Following this, out of curiousity I looked inside by Beogram 4500, 8500 and 9500, which I know have essentially the same mechanism.  I found in each case that the clearance between the platter wheel and the wire drive pulley is very small - probably 0.5mm if that. However, on my Beogram 3300, I am still unable to figure out why they are meeting each other when the arm is moved inward.  I have been unable to find any way to adjust it.

Check that the tonearm measures 53mm above the top of the surface to the top of the arm. Also check with the carriage moved to the center and the platter off that there is 2.5-3mm of between the top of the subchassis and top of the turntable surface all the way around.

Thanks for the tip.  Yes both of those measurements seem to be right.  I had already attempted to adjust the platter height as described in the service manual, but it makes no difference to this specific problem as the whole mechanism is either lifted or lowered, so the distance between the wire drive pulley and the platter wheel remains unchanged. It's as if the wire drive pulley is lifting just a little bit as it brings the tonearm to the centre of the platter.  It is just about making contact with the underside of the platter wheel.  I cannot see any way to increase the height of the platter (independent of the mechanism) or to lower that wire drive pulley. If I apply a little bit of force to the wire drive pulley while it is making contact, then spin the platter wheel, they come away from each other and the platter wheel moves freely, but once I move the tonearm out and then back in again, the issue instantly returns.

Replace the washer under the wire wheel with a thinner type. Martin

Thanks Martin - I'll take a look at the weekend.