They cover for the first 3 weeks, this is now week 4. They did say it was my fault, shock of the move etc but the vet disagreed. Thinking twice about going back there
i would get the vet to give you a receipt for all the treatment & consultations make sure its dated & itemised from the first treatment they gave the guinea. take copies & take the receipts in to the store, ask to speak to the manager or supervisor, etc. Also take a copy of your receipt from the purchase of your guinea & any equipment purchased (disinfectants, log cabins/wooden toys etc...will need replacing due to fungal spores)
just point out that it was within the first two weeks that he had veterinary treatment, you bought everything in good faith on their recommendation & sales.
if they still object to any refund for treatment...
then try this link below

basically tell them they sold you an animal contravening the animal welfare act 2006
*(e)its need to be protected from pain, suffering, injury and
disease.
also they look like they are on dodgy ground with the
*(d)any need it has to be
housed with, or apart from, other animals,
by only selling you one, they are social animals & should have found out his housing circumstances prior to selling you him.
http://www.legislation.gov.uk/ukpga/2006/45/section/9
also mention that you will write to your local council that deals with licencing the selling of livestock on the premises & see what they have to say about it....
this i'm reasonably sure will work in favour for a refund & maybe a complimentary bag of hay for your trouble/compensation!
if the shock of the move caused it then why do they sell animals in the first place? every animal they sell is moved ---- dumb arses!
Try finding a friend for your handsome piggy when healthy - from a reputable rescue, have a look on the recommended rescue locator above, they will sort out a compatible friend for him that is healthy, properly sexed & give you support for the duration of your piggies life.
good luck & let us know how he gets on

Speedy recovery. xx