Matt I personally wouldn't attach a harness to a cat ladder as it's not a solid structure, that would be the HSE line anyway- it would doubtless work but they would not accept it. Scaffolding is going to cost more than the roof clean as for the HSE inspection it will have to be certed'. I reckon you can make it safe using a rope access technique-IRATA- (harness and gear for this costs a few hundreds) but again this requires a lot of training etc. I don't think HSE work as much on Sundays!!!!