I think that you should have listed losses in the year that you incurred them, yes. But I don't think there are any penalties. I'm not 100% sure on that point though (the obligation to list losses that you incurred in previous years) but there's certainly a box for it in the CGT form. I'd just set off the losses and write them a letter with some of the details (amount of loss, year incurred) and say that you neglected to include these in previous returns as you didn't realise that you had to provide that detail if there was no CGT liability. I wouldn't go so far as to actually fill out returns for those years - unless they ask, of course!
Can't see that there'd be any penalty but they may ask for background paperwork (and I think that's even unlikely).