TV drama one-off I wish to recall

[ClareA]

Does anyone remember an excellent TV drama of some years ago in which a hospital porter comes across the abandoned belongings of a doctor in staff accommodation as he's clearing the room out, and thereafter assumes the doctor's identity?
Parts were of course implausible, but all in all it was a cracking yarn.
There's a significant scene in a swimming baths when a former colleague recognises the imposter.
It was based on a novel by a doctor, and I'd also love to know the name of the lead actor, as he's been in many dramas but I can't recall his name.

Many thanks to any who can help.

[JackDanielsU]

I think you need

a 1990 film called Paper Mask starring Paul McGann

Based on the book by John Collee

[ClareA]

Well, thank you both, housemouse and JackDanielsU. Much appreciated!