Thankfully managed to source a like for like ballast on amazon 16 quid. So hope that resolves things.
More concern is how the water is getting in. It must be running down the pitched roof soffit boards and entering the light fitting through the hole that the power cable comes through. Not sure what the hell can do about that though..... Will suck it and see if happens again..... Cheers
I would have simply replaced it with a modern LED unit, it would not have cost much more, but...
The secret with mounting any electrics outdoors, is to seal it as best you can to prevent water ingress - but to make sure water which does get in has a means of escape. I always ensure there is a small hole of two, at the lowest point.