You dont have a spur you have 2 20 DP switches and these should be dedicated circuits from the CU so I am guessing that your immersion has 2 heaters top and bottom ie top is the boost and the bottom is the standard heater. Is this correct also thanks for the pic but not trying to lecture you but more information is required.

As for a timer I think Horstmann Economy 7 Quartz Timeswitch may do it. tried to put in hyperlink but it no worky worky

Forgot to add that this unit should be wired to the bottom heater plus you can overide it whe you want ie best of all worlds plus it is simple to wire

Hi Oldtimer,

Yes - you are correct. 2 heaters, top and bottom.

The Horstmann product is £66 - quite expensive.

I have just found a cheaper product:

http://www.amazon.co.uk/Smiths-Time-Guard-TS800-Timer/dp/B0036OXD0G/ref=cm_cr_pr_product_top

Would it be possible to convert the red circled switch to a standard plug, put the above timer in, and put a plug on the end of the immersion heater cable to plug into the timer?

Me thinks you should get the Horstmann yes dearer than the "plug in timer" and probably 100 times safer remember cheap is not always the best

generally it is not recommended to have your immersion heater on a plug and socket if you can avoid it.
An alternative product that is easy to fit in the same space but is about £40 is the timeguard supply master fused spur. The unit does more than you want it to but could be a good choice.
24hr Fused Spur Timer by Timeguard