Having taken the oven out to fix a problem with the hob (720EF double oven and grill), reinstalled the oven and powered –up, the clock won’t come on and stay on, the oven and hob appear to work! Any suggestions about what the problem/fix might be?

Both the top and bottom oven will only work if a time (any time) has been set on the clock. Setting the time operates a contact to switch the power through. The only exception is the grill. This will work even if the time has not been set. So if the oven controls/lights and heating work, to me it means that the clock/timer is half working and the only thing that isn't is the display.
I haven't had the clock/timer to bits but a fellow blogger has said that in some timers there is an electrolytic capacitor that controls the display. When this fails as they are renowned for doing, the dispaly mal functions. If there is one then Maplin have some - VHR41W/43W and DT64U depending on value etc. Let us know how you get on.
If you need a new clock/timer they are around £80 from espares.