Being on call should just be paid at the normal hourly rate. It's an outrageous setup up: we need you to available but don't want to pay you for it.

I did it for a few years and it made me pretty depressed. By the end of it I stopped giving a shit and just resumed my normal Friday night of having a few beers (or a lot of beers). At first they offered me a rate that was less than min wage. I said no until it was an acceptable rate. Every other company seems to pay terrible on call rates in the UK. It makes me angry just thinking about this period of my life.

Seriously, if a company wants to make money doing SAAS then they can't expect to steal employee time. My advice, if asked to do it, refuse until they offer very good money for it. Good luck to them recruiting a system expert to replace you.