Can anybody point me in the direction of a site that shows how many spousal applications there are in the UK each year and the success rate?

Immigration statistics are published quarterly by the Home Office; see Immigration statistics quarterly release

These documents, although aimed primarily at child settlement applications, may give some information that you are looking for. And they are, of course, interesting in their own right :

CC_Annex.pdf

Ministerial_response_to_the_Children_s_Commissioner_for_England_on_the_minimum_income_threshold_under_the_family_Immigration_Rules.pdf

Referrals_to_the_Referred_Casework_Unit.pdf