A good end to the story . Thanks to Phil Crouch of CPC who was nursing my Alpina through the SLS pipe change, I now have a new MOT - and I didn't have to change the SLS units. IT would seem that after sitting for an extended period, some oil did get past the top seal of the strut and looked wet and messy - bad enough for MOT tester to issue a fail and claim excessive leaking...
After a couple of days of being moved in and out of Phil's garage while I sourced info on replacement struts, Phil noticed there were no leaks on the ground and no fluid was being used and the car held pressure as it should. The strut was thoroughly cleaned and monitored over a few days and apart from a light misting on the insert, no fluid was escaping - it looks as if its in good shape after all.
Got it re-tested and a pass certificate was issued, which is great news. I'll keep an eye on it over the coming days/weeks and months and if it doesn't leak, I'm not going to change it.
What we did have to do and worth mentioning for other BT owners, was attend to some corrosion to the floor of the car above the CATs. My heashield was in very poor shape and in fact fell off last year - I bought a new one and asked Phil to fit it. Trouble was, in the meantime the heat from the CATs accelerated corrosion and a small hole was appearing. A plate was made up, the rot cut out and all welded to perfection - protected and covered by a shiny new heatshield. Check yours too.
Thanks to all who chipped in with help and advice, much appreciated. I drove it home this evening and it felt fine, felt great in fact to have full boost on tap and it still goes like stink