I read in an interview with Nick Seymour that Tim was supposed to be a full member, like Mark Hart became some time later. Nick also said that he would have been more comfortable if Tim had been a one-off guest for the Woodface album (and tour).
At some point Neil and Tim were doing almost everything while recording Woodface, at some point replacing Paul Hester by a different drummer, Ricky Fataar.
I can't remember from which source this was, I think it was a interview from 1996... (mojo magazine?)