Your basic protection is contained in the sale of goods act 1979 as amended, and possibly in the distance selling regulations (consumer protection 2000), where you have purchased without the opportunity of inspection perhaps from a catalogue, magazine or web-site, in which case you would have seven days in which to reject or accept the goods.
If the goods do not come under the DSR, they must still be of satisfactory quality (since the 1994 act satisfactory quality rather than merchantable quality), be fit for purpose and match their description, if they do not you are entitled to reject them, the reasons given for the substitution by the provider are not acceptable. Write and demand a refund which if not received within a reasonable time, say 30 days, you will commence action in the county court, write again after the 30 days and head your letter “Letter before legal action” and give them a further 14 days, if they have not made a refund commence action in the county court.
It may seem you are giving them a long time to pay but English law likes you to be reasonable. Keep copies of all letters.