Thanks guys, came back to look at thread last night, but mums net off line for the 10 min window I had!
He was brill! so excited and yet managed to only watch for 'one open and one shut' of the automatic doors, so proud of him! (been working so hard on that).
They did what I expected, I said does thbe worker they have just got for him use Signalong or SSE... (been very clear that ds uses SSE, I know lots of overlap, but he has 200 plus signs he uses and understand more)
They did the old "we have done courses and have books" response, which is of course not much good if you haven't used if for a couple of years, and can't run off mid session to look up what ds has just signed to you.
But am taking pragmatic 
sneaky approach, will go in with his 200 signs and 'assist' their learning, knowing the statement which is at prov stage will say 'workers must be fluent in BSL, able to interpret his signed replies and work wit us and SLT to advance his signing and speech..
Feels ok to start low key as their general attitude good, and he is only there 2 sessions a week, and also should be going there for school (named on statement, hopefully!) so want to build up good relationship, (can go to scary stropster later) 
Really appreciated your responses, was so nice to know others out there while i 'wobbled'
Thanks!