The text of the book doesn't explicitly state that the invitation is issued, but the final illustration, by Boston's son Peter, shows Mrs Oldknow's blotter:
Mirror writing is always an invitation, but to save you having to find a mirror here is the same image horizontally inverted:
How do you read that? It's pretty clearly Queensdale Rd., London W11, but what's the number? I suggest it's more like 18 than anything else, but it looks a bit odd to me. It might possibly be a 16, or 1B. The '1' also looks rather strange.
Queensdale Rd does indeed exist. There doesn't appear to be a 1B (at least at the present date), but here are 16 and 18, either of which you are free to imagine as the address of the rather eccentric palaeologist, Dr Biggin:
The other question, of course, is why Peter Boston chose a real address for this fictional character.