I like the pads (like care mats/inco pads but washable). They don't cover the whole mattress but you can put them where the wee is most likely to go. They are heavy enough to stay put, and they are quite absorbent so they contain the wee iyswim. We bought them from Boots but I can't see them on boots.com any more.
We have also tried the pvc ones and they are just too rustly, also they don't absorb the wee so the sheets end up soaked. And we have a few Hippychick flat ones but they aren't big enough imho so they do end up shifting around. We tend to keep those on our bed (for midnight visits) and use the pads for the boys.
Can you guess I've had many years of nighttime accidents?? Even with 'pajama pants' (DryNites) we often have leaks. Haven't tried tie/fitted ones but can imagine the ties would be annoying.
John Lewis have a good range. I got my son's fitted waterproof sheet for his new single bed from there- it's polyurethene with a towelling top cover so it's not "sweaty" in bed; I put a normal fitted sheet on top. They also do quilted waterproof sheets and jersey waterproof sheets in all bed sizes. They're priced from £21 for a single size.
I have had a waterproof "pad" in the past but found it rucked up under the sheet.
Try Dunelm Mill, they have fitted waterproof, breathable cotton ones from £7.99 for a single-however they do take an age to dry(we dont have a dryer)so we have 2 on the go, and ds nappy leaks most nights but thats another story!
I have the mats. I used to put one under the sheet and one on top so if ds had an accident in the night I just whipped off the wet top mat and he could get straight back into bed with the other mat in place under his sheet.
I had one from Jojo that has side panels to tuck under the matress and 2 others but I'm trying to remember where I bought them from. I think it was Boots.
I also used the disposable bed mats if we were going to stay somewhere else and they worked well too.
I have the hippychick ones and would def recomend them. Ds is VERY sweaty and these just breath so well. I have used it as a gereral protection until now just to save the mattress but we are about to try night training <<eek!>>
My plan (which I have nicked off someone on MN so thanks to you!) is to put the hippy chick one on the mattress at the bottom then layer smaller pads over the place an accident is most likely to hit with normal cotton fitted sheets. So it would go mattress, hippychick fitted sheet, waterproof pad, fitted cotton sheet, pad, cotton sheet, pad, cotton sheet etc. Then if he has an accident I just have to whip the top cotton sheet and pad off rather than having to change and put a fresh one on.
I have some washable pads which are great if a little expensive from here I used them to cover the sofa when ds was pt in the day. Also have afew packets of the disp pampers ones too.