Don't live near there, but your local tyre dealer should be fine...look to where all the farmers go. (not kwik fit !)
You can look on ebay for ideas of prices, but I would buy locally so you get fitting included...and they'll probably come to you to fit if its back tyres.
If you google agricultural tyres Yeovil loads comes up.
You do you suddenly think you need tyres urgently ? You only bought it a few days ago and it was alright then ? Tyres don't suddenly become illegal on tractors like they do on cars.