Slobby,
If you have that much Multi Crew time then you can claim a credit from the CAA for a MCC course.
They require over 500 hours multi crew time. This time can be in a SPA, as long as there is documented evidence that this aircraft was required to be operated exclusively by two type rated pilots, even on non revenue and positioning flights. Normally this is listed in the AOC somewhere.
Now, if you are going to operate a SPA as a multi crew aircraft then it needs to be signed off in the AOC. If this is not done, then as far as the CAA are concerned the co-pilot is sat there with there thumb up their arse doing sweet naff all.