I *think* the problem is sorted.. dad cleared the drain area under the wheel arches... LOTS of mud.
Interestingly though - the water was also getting in somewhere else too.. there were rivet holes that seemed to go straight down under the sunroof into the headlining. So these have had some sealant put on them. Just gotta wait and see now if all is good. He also took the rather wet and stinky foam backed lining out so that can dry off properly.
Just gotta wait for the next rainstorm now to confirm it's sorted - but it's looking very very good!
I'll probably write up a long 'how to' with photos to help others!