I didn't realise two screws were hidden by the steering wheel, easy when you know! I was actually only wanting to get the housing off to get access to the clock spring which I'm hoping will arrive tomorrow. I thought that if I asked for info for that I may not get any replies, it's apparently not a very common issue with a Lexus. Anyway, it's all in bits, all I've got to do now is get the steering wheel off the splines!
One thing I've learned from this is the importance of letting the capacitors in the air bag loose their power for a good 15 minutes before removal. I've not had to learn the hard way I hasten to add!
£244 for a new clock spring from Lexus, hopefully my German purchase for £30 will be OK. The horn not working was the only reason for an MOT failure, more than happy with that!
Thanks again for the help.