It wasn't unusual for non-members to be in the office, particularly after a demonstration. The office had two floors and non-members would be in the larger meeting room. It wasn't a security risk, and there wasn't much to look at anyway. The famous filing system was a joke. The accounts of people moving into other people's apartments, above a restaurant etc. could only come from an established if not senior member. This kind of detail was not recorded.
Tompkin's statement is over 50 pages long and very muddled. He disowns quite a lot of the reports attributed to him, says he couldn't have handed over addresses because his relationship was too distant to get this info (sometimes he implies differently), and often confuses the Sparts and RCP (by which I mean any of RCP/RCT/RMT/RLL; alphabet soup!). I don't think the Sparts had many East London events beyond a few paper sales. The RCP were active in East London with their front organisation ELWAR, which he mentions several times and at one point says the Sparts sent him into it (didn't happen). At times he claims the Sparts and the RCP were close organisations, he even uses the phrase "sister organisations" to describe the relationship (which couldn't be further from the truth).
He's created a narrative from muddled memories - no shame in that, we all do it, but the report is a sham which provides no real information beyond the fact that someone was passing info to the cops.
Now you've jogged my memory. I recall a young guy (younger than the rest of us) who habitually wore a shirt and tie and always seemed to be grinning. I don't remember his name or the expulsion. He took up with an older woman, sometimes they'd sit on the stairs locked together in a passionate embrace. Is this who you're thinking of?
Barry Tompkins was a van driver with a heavy beard apparently.