To be pedantic, your local Co-op shop hasn't been taken over by Spar because Spar don't directly operate any shops. It will be an independent retailer that's taken on the business and who has chosen to sign up to selling under the 'Spar' brand name, thus committing to buying stock from Spar and honouring their promotions. (i.e. Spar is a so-called 'symbol group', operating in the same way that Budgens, Londis, Mace and Nisa do. They don't own any stores).
So your contract was with the owner of the shop, not with Spar, and its that store owner that you should have complained to (and sought redress from).
I doubt that you'd have had much better luck getting any form of compensation from the your Co-op anyway. My experience of moaning to my local Co-op (e.g. about sending out discount vouchers so that they arrive on or after their expiry date) has never got me anything more than a very half-hearted apology.