Hi Lady F - I went to a great showroom . Ended up forking out £1300 for a decent quality one - Elite T9450 HRT Folding Treadmill. As you mentioned, it is important to get one with the right weight limit. also, you need a decent motor on it, otherwise when us - ahem - larger people walk/run the belt struggles to turn and can judder a bit. The one I got works great for me, just need to use it more often!!!
I bought from a company called fitness superstore - had to swallow my pride and go to one of their show rooms (I went to Northampton). Took my boyfriend for moral support - but it turned out great as it was a deserted showroom in an industrial estate, but they had about 20 different treadmills and a really helpful sales guy who explained the pros and cons of each and encouraged me to have a Go. Was even very kind about explaining the weight limits etc and made me feel really comfortable.