So my son is moving on up from his cot to a bunk bed. For now I need a cheap double bed mattress for him down the bottom. I want cheap because I am sure over the next 2 years there will be some accidents in that bed.

Plus he's only with me 2-3 nights a week so he doesn't need to use it all the time.

So all I need is something cheap but ok for a 2 year old to be comfortable on.

What ^ he said. You don't need anything flash for a 2 year old. Any stock standard mattress with a decent protector will suffice.

Enjoy your son, take lots of pics and video, it will be a blast.

I suggest going to a mattress convention and getting him to try everything he can. Make sure you spend top dollar on a mattress. The more you spend the better your sons back support will be and the better his life will be. All the sales reps will tell you that. They have no reason to lie.