It's possible I could go at the end of next month, but I wouldn't be able to stay longer than a week, so it's more likely to be in the summer.
I'd strongly suggest coming at the end of next month: come summer temperatures will be over 30c and most places outside Warsaw do not have a/c. I think that the new regional trains have it but I it might not work well enough, your TLK train to Lublin almost certainly won't have any a/c. Also, in the Sobibor area mosquitos can be a real pain in summer.
I would be flying into Warsaw and out of Rzeszów to avoid costs associated with travel to and from Modlin airport. This means that the last city i would visit would be Lublin, as trains from Zamość to Rzeszów take a lot longer.
Can you not fly out of Lublin airport? Rzeszow airport isn't exactly city centre, so you're be looking at about 50zl to get there from the station (Warsaw to Modlin is 99zl fixed fee with Glob taxi). Also, from memory there are only about three trains per day from Zamosc to Rzeszow and they can get very very crowded. I'd fly from either Lublin or Warsaw or Modlin.
I think 3 nights in Warsaw should be long enough for me to see everything and I would then get a train to Zamość.
Three nights should be enough for Warsaw. The don't miss stuff is MDM and south central Warsaw (afternoon/evening you arrive), Old Town and New Town (a longish morning), the Royal Route and Lazienki (the afternoon of that same day), Uprising museum (a morning) and the ghetto (the afternoon of that same day). The train to Zamosc is one of the best trains in Poland, but do get the one-change train which leaves Warsaw at about 11 am, the morning and evening trains can get crowded. Spending the extra 20zl for first class to Lublin might be a good investment. If you have to change in Rejowiec you can expect that train to be old and crappy, the ones which go direct from Lublin to Zamosc are far better (they're about two years old).
How long should I allow for staying in Zamość? If possible I would like to visit Bełżec from there, so how long i stay would depend on if this is possible ( no idea what transport links are like as i don't want to hire a car ). I would probably not be arriving that early in the day either as it's a longish train trip.
You can see Zamosc in a day (it's very beautiful but also pretty small). Belzec could be a problem to get to. I'm pretty sure that passenger trains no longer run there and I'm not sure about direct buses (you may have to change in Tomaszow Lubelski). It's about an 80 km round trip, so you might be able to convince a taxi driver in Zamosc to take you there and back (with about 90 minutes of waiting time there) for 150zl or so. Alternatively contact Tourist Info in Zamosc and ask them for advice. The museum at Belzec is small but very good, don't miss the map showing when each village/town/city in Poland lost its Jews (the lights in each place go out as the months go past until there are no lights left). Belzec is by far the most powerful of the death camps, so I really do recommend making the effort to get there. I'd suggest two nights in Zamosc (with sitting in Zamosc Old Town square the best way to get your head back together after Belzec).
How feasible is a visit to Sobibór from Lublin? Again is this possible to get to by using public transport?
I wouldn't like to do that as a day-trip from Lublin. It's about a two-hour drive from Lublin to Wlodawa, so allow more like two and a half for a bus, and then Sobibor camp is about 12km south of Wlodawa. You'd need to get a taxi from Wlodawa to Sobibor camp (note that Sobibor camp is not in the village of the same name) and then make sure he waits for the 90 minutes it takes you to walk round the place. The museum at Sobibor camps is tiny and not the best, one time I was there it seemed to contain more dead flies than exhibits. But the camp is worth visiting (with a guide-book and a map), it's the only one of the Aktion Reinhard camps where you can very easily make out the lay-out (at the other two post-war memorials stop you from doing that), also it's the one where there's most still left (the commandant's house is still there, the punishment cell for SS guards who misbehaved, barbed wire still in trees, even the very ramp which was used to unloaded the cattle trucks).
Wlodawa itself is nice enough for a small town and has a rather nice synagogue which now contains a regional museum. I suppose you could possibly get a bus from Zamosc to Wlodawa first thing in the morning (that would take about two hours), visit Sobibor and then go on to Lublin. Either that or do a night in Wlodawa, apart from anything else the place is cheaper than chips, I think I paid 80zl for a perfectly acceptable single room with private bathroom.
As Majdanek is on the outskirts of Lublin I will definitely be visiting this camp, so am really unsure whether to allow for 2 or 3 days In Lublin.
I think that you might find you're 'camped out' after Belzec and Sobibor. Majdanek frankly needs a bit of work doing to it; I'd certainly suggest Belzec and Sobibor over Majdanek. Lublin is a cracking city but you can see most of the touristy bits in a couple of days. Maybe one day for the old town and 'new town' (plus the only shopping mall I know of which was built on the site of a concentration camp, and by an Israeli firm too) and then the next day for Majdenek (if you insist) and then a walk round the Aktion Reinhard traces (there are some crackers, including the house where Globocnik lived and a building that used to be an SS casino and brothel).
Finally, is there anywhere that i really shouldn't miss seeing that is relatively close to either Lublin or Zamość? No more than an hour away by train.
There are a few places worth a visit but limiting yourself to train only makes them far less attractive. Do you really not want to hire a car?