What is the way to prevent being gazumped like this? Does any one know of any way to deal with this?
Quotes (oral or written) for the small projects you mentioned aren't necessarily a guarantee.
Therefore you should clarify with them beforehand if their prices are only an estimate or a guaranteed final price.
If they say it is the final price then tell them you agree and won't pay any more once you accept the work was done satisfactorily.
Also let them know that if the work requires more supplies or time to complete than they originally factored in then they should inform you before continuing so you can agree to it or not.
Another thing to consider is if they told you when presented the bill that there was a need for extra time or materials than original thought then are the final costs reasonable. They should be able to demonstrate to you what extra work was done. For instance, if they needed to replace a pipe and when doing so more of it broke to due age and deterioration then yes costs for the extra time and materials may be reasonable to complete the job. Your position shouldn't be that they gave you a quote but when the work was more involved than anyone could have realized that they should essentially do the extra work or use extra materials at no additional cost to you.
And always inspect the work. If they said more was required but it is obvious it wasn't done then tell them you will only pay the original quote.