Rob, I have one of those Clarke guns too. Not a bad bit of kit and no money at all really. In terms of battery powered ones, you get what you pay for. The Milwaukee and other top pro brands really are built to last and frankly I don't think I'll ever wear one out in a DIY setup. The battery technology now is so far removed from days of old and manufacturers are very aware of concerns such as yours. You can get the LiION batteries in the M18 Milawaukee range for as little as £25. Having collected this little group now all with the same batteries, I'd imagine I could cycle them round and never need another one.
How would I rate the corded Clarke against the battery Milwaukee? You know there really isn't much in it. But if I had to say which I think would pack up first in a head to head test undoing lorry wheel nuts in a test, I'd say the Milwaukee would still be going strong long after the Clarke was on the bench in bits. But then that isn't what we do is it.