Thanks for the replies everyone, I will look into the durex ones. I tried a few different ones tonight, included in the pack I had with the pump was a clear gel one and that seemed to be the best one, if a little tight. It seems quite hard to get the balance between comfort and sustaining an erection.
Lyn - I had no nerve sparing and expected to have no erectile function at all after the op. This was true at first, no amount of physical stimulation seemed to do anything for me. However after some weeks I seemed to get a little response. Then I was given viagra to try and on second use I actually got a semi erection. It wasn't enough for normal intercourse but it was certainly encouraging. Bearing this in mind - yes I think you may have a point there. When I release the vacuum my erection does reduce immediately, but I do hold a semi-erection for about 5-10 minutes.
Another question if I may, all instruction says that you must take the ring(s) off after 30 minutes. However I cant see any info on when you can use the ring again afterwards. Do you have to wait fr a while or can you 're-inflate' straight-away and start again?