Hmmm - I'd recommend checking around before getting too carried away.
Just checked on a Mann oil filter and they wanted £20 quid, then with the discount it comes down to £7.50, which is coincidently about the usual price from everywhere else.....
Euro do the same tactic, have more sales on than DFS, but always works out about the same price at the end of the day. Sometimes it is still a reasonable price though.
It must vary quite a bit, the Bosch filters for 1.4tdi Polo are £6.19 on Autodoc compared to £2.96 with the GSF deal for exactly the same unit
Just checked for my V70, Mann filter is £10.94 on Autodoc and £7.04 at GSF
Just checked 5 other cars and Autodoc was more expensive on all of them, sometimes it was only a few pence but mostly multiple £ and with the filters I'll be buying I will probably save £50-60 over Autodoc prices.
There is a GSF just round the corner from work too.