Use code JAAFF to get £10 off any purchase. There's a sale on with many shirts starting at £19.95, so, with delivery of £4.95, it's £14.90 delivered. Topcashback are also giving 10% for three days only, but not payable if using this voucher code.
Edit: Some shirts priced at £17.95 so £12.90 delivered with code. Thanks for MrWani for spotting that.
UPDATE: Code no longer appears to be working online but thanks to Stewamar for suggesting calling the sales line where it is being accepted.
Code expires 20th June 2017.