OK - I fixed the issue :-)

Upon disengaging the drive reel brakes, and disengaging the drive cog, I found both drive reels were stiff to turn manually. So for each drive reel: I removed the top circlip (which is microscopic, be very careful here!) then removed the drive reel components. I cleaned the drive reel shafts, and the drive reel bores with IPA. They all had evidence of dried lubricant / grease. I re-lubricated the shafts / bores and reassembled everything. The most difficult part is reinstating the circlips, which requires a very steady hand, and steadfast concentration. Upon reassembly, it was immediately evident that both reels were easy to turn manually. I powered the Beocenter up, and it's now working fine! Play in both directions, FF & rewind are all operating as they should. I did notice the left drive reel was skipping a little when winding. This was more audibly obvious than visual. This was rectified by bending the drive cog metal stop away from the drive reel. I made three tiny adjustments followed by testing, until it was winding quietly with no skipping.

All sorted :-)

I have a Beocenter 8500 with the Mk1 tape deck, and mine has a similar issue (even after I replaced the belt).  This sounds a bit complex and I'm not sure I want to risk the attempt, but thanks for the info anyway - I may give it a go.