Gents,
I removed the blower fan and motor and shoved my hand up inside and sure enough there was something like a concertinered fabric filter. I got the vac and taped a length of 15mm foam pipe insulation to the hose as the bend is too tight for a normal pipe. I then waggled the thing blindly around in the cavity trying to make the end hit the filter.
Afer a few minutes I withdrew the hose and replaced the fan and hey presto we have airflow !. Even the two vents in the middle of the dash have air whereas before they were dead.