I went into comet to get such bits before going to america (use it in the hire car :) ) - they sold a suitable PSU (12v, 1A, 2.1mm centre positive - actually, it was a "one fits all" adaptor which had the right plug).
The tape adaptor is a bit more tricky, you need a phono-to-3.5mm jack (eg, PC soundcard output cable) and a barrel 3.5mm-3.5mm adaptor - the tape adaptor will then plug into this.
Down Tottenham Court road, you should have *no problem* whatsoever. Every time I've been there (the last 15 years at least) I've always found some gadget to tempt me - they even had in-car DAT players for 300 quid last time I was there. Someone had found a warehouse of the things (unsold due to them costing about 1500 quid new) and was selling them off. Major 80's plastic design, Blaupunkt and JVC...
Don't forget to haggle :)
Hugo