"It was used by the royal marine reserves during the 80s and 90s not sure if they are still using the place"
jimbob thanks you've just confirmed that I'm not losing my memory:
When I was about 8 or 9 I went on a cub scout trip to see the "SBS" and I could of sworn it was in Birkenhead docs. We got a ride on a boat got shown scuba gear and guns etc - I loved it but was beginining to think that I'd imagined it all. The SBS having a base in Birkenhead docs - what nonsense - but it's true (nearly)