increasing or decreasing in numbers?
Poles don't come to GB and Ireland anymore. Emigration stopped around 2008. Now those who managed to set their lives in GB stayed there for good and those who don't are returning home. And THIS IS A FACT!
From my point of view ive never heard of any Poles that are going to, live in or have left Wales.
I've read an article about Wrexham where live many Poles. Once I've met a guy from Wales and he said that in his town there is lot of Polish people.
I guess there are/were many Poles in Wales but Wales was not so popular aim of emigration (as Scotland, England, Ireland). I have no idea why.