Personally, it's of my opinion that unless stated on the invoice, agreed before sale, mentioned verbally etc. that the .uk was included, then it simply wasn't. That's like buying a birthday cake and saying "Oh but birthday cakes need candles, where is mine?" even though the candles are on sale for £2.99 and didn't come with the cake. Secondly, this whole "I have the rights to the .uk if I own the .co.uk" ended in July this year. Furthermore, it's irrelevant because the .uk wasn't included in the sale.
tl;dr - The buyer of the .co.uk has no chance and under DRS I think they would see it the same as well. Make sure you can prove email conversations, screenshot stuff, show invoices etc. if there were recorded phone calls get them up too. But I think they have no grounds. Unless they have a trademark on the term, which (because you stated it was a generic domain) I don't think they will have.
Also, nice sale price man. Well done.
EDIT: Just noticed the sale was in March. What I said still applies and quite frankly, I think once you have executed the registration on those rights to the .uk, the domain becomes a separate entity as someone else mentioned above I think.