Thanks to Artic and Trikey. Today I completed the job of re-joining the passenger and drivers side sunroof drains. I used PU pipe kindly supplied by Trikey. It does away with the need for pipe connectors that I had used before and failed. This time I used CT silicone sealant that is up to the job of sealing in both wet and dry conditions- can even be used under water. To finally check that the joints were water tight I used a small piece of tissue paper around the joint and then I poured a couple of litres of water into the sunroof area. No wetness was detected on the tissue. All 4 drains in all 4 wheel arches had a stream of water coming from them. I'm confident that this has cured my leaks. Now for a new stainless steel exhaust.
Check out Arctic's how to on front and rear sunroof drains.