While checking out the hum issue discovered that the tape deck metal chassis is floating/not grounded and the metal work is very sensitive to be touched. Could see no way that the chassis would connect to ground. Tried adding a temporary ground connection which only reduced the hum level by a small amount, so removed the ground connection.
Refitted the chassis and reconnected the heads wiring. Hum level now probably around 50% lower (chassis still not grounded).
Resoldered quite a lot more of the various component connections particularly ground connections (none looked bad). Hum now appears fixed (or at least has gone away for the time being). While doing this noticed that the main drive belt did not look like it was position correctly and found that during the refitting of the chassis had got the belt hooked behind an electrolytic capacitor.
Unhooked the belt but the belt the fell off the driving gear and I could not get it back around the gear, so had to remove the chassis (which is not as easy as it sounds) to be able to relocate the belt correctly. Refitted the chassis and then found that the tape would not play and found that there was quite a bit of tape tightly wrapped around the pinch roller.
Had to cut the tape to get the cassette out but could not remove the tape from the pinch roller in-situ so had to remove the chassis again ! getting good at this now
Not sure how the tape got wrapped around the pinch roller, it must have occurred during the hum tests (I had forgotten the cassette was even fitted) due to the belt being around the capacitor.
All back together now and working well with no hum. Next to check line recording from the DIN socket and try and fix the tape counter reset mechanism.