Hi Nigel,
I have a Mk2 gs300 - year 2000 - I believe it will be the same.
The ashtray simply pulls out, grab it and pull it (lol). It will pop out, trust me, there will be no damage. Next, remove the air vents at the top. To do this get a cloth and a biggish flat head screw driver. Wrap the cloth over the end of the screw driver and use this to prise this out. Do one end, then the other, again, this simply pops out. Take care not to damage the dash while prising it out.
The heater and headunit are one unit. There are two bolt head screws at the top and two at the bottom, the top ones are shiney goldy colour, the bottom ones (under the stereo unit) are black. Un-screw these with a socket (and remove) and the head unit/heater control block can be pulled out easily.
Fitting it back is a reversal of the above. Its quite simple mate, don't worry! :)
Cheers
phil