Hi Ben. Thanks for the advice.
The tapes and tape unit had been untouched for 26 years. It did most definitely exhibit sticky tape syndrome when I first managed to get the unit operational. When I first experienced the increasing sluggishness and even slowing down when just playing tapes I searched the internet. Thanks to various posts on this forum (probably many by yourself) I learnt about sticky tape which explained why I was getting so much debris being dumped on every contact surface. I've used the same tape to get the unit running and it seems to have stopped depositing debris so I'm happy it's no longer the cause of the slowing down. I do, however, have loads of tapes I'm going to listen to and suspect that it's something that will haunt me for the foreseeable future.
The slowing down at the end (about 1cm on supply) during F.FWD is really only an irritant but it needs sorting so will still give the clutch a go (I like things to be right). I will also look at the various contact points and see if I can rotate them as I hadn't thought of doing that so many thanks.
Richard.