http://www.appliancespares2go.com/department/clock_timers_7/
At £70 they seem very expensive - shop around using google.
There ae dozens of Rangemaster clock problems on here - oven(s) will not light. Pages 10 and 11 of this PDF show how to operate the clock.
http://www.rangemaster.co.uk/SiteImages/Site_311/BigMedia/ClassicDeluxe110DF_06-06.pdf
My guess is that the operating instructions are so complex that clocks are being replaced when they don't need to be - either that or the clocks are badly made / designed - a cooker clock failure should be a very rare thing - and to charge so much for such simple electronics is very naughty. I would guess the manufacturing costs of the clock to be less than £2. They are charging mobile 'phone prices! Scandal.
Peccavi
October 2008