That is why cheques are not an authorized payment method on eBay Canada or eBay.com
Even if you had opened a dispute within 45 days eBay could not have helped you because you used an unauthorized payment method.
I understand cheques are permitted on the UK site but your banking laws and consumer protection laws are different.
Has the cheque cleared yet?
If not go into your bank and put a stop payment on it.
Because you are in the United Kingdom you can chat live online with eBay UK customer support by using this link.
However I doubt they will be able to do anything for you.
http://ocsnext.ebay.co.uk/ocs/home?