Mothercare have a price match promise where they'll match a competitors price and if it costs over £20 they'll also give you a £10 Mothercare voucher.
www.mothercare.com/Price-Promise-Other-pages/b/330387031
This carseat is on sale at Amazon for £24.99. www.mothercare.com/Britax-Adventure-Car-Seat-Billy/dp/B004EYHV3M?ie=UTF8&ref=sr_1_1&nodeId=42764041&sr=1-1&qid=1307436132&pf_rd_r=0SNTDYR3S5TMF4J933M0&pf_rd_m=A2LBKNDJ2KZUGQ&pf_rd_t=301&pf_rd_i=0&pf_rd_p=231490887&pf_rd_s=center-7
I can confirm Mothercare have just price matched this for me and will be sending out a £10 voucher. Delivery isn't included but you can collect in store free in some stores, so this is probably of benefit to you if you have a Mothercare collection store close by.
Hope this helps someone!