Millers is ~£2.95 a bottle, and you stick the lot in prior to filling up. The neck is narrow enough to fit inside the opening to the petrol tank, so you just stick it in, let it empty, then fill up with your petrol.

However, regarding all this talk about Optimax/SUL/Ultimate. You stand more chance of achieving a higher octane using Optimax/SUL/Ultimate + Millers than 95RON UL + Millers. And if you have an import, it's even more important. You may get the occasional bad tank of Optimax/SUL/Ultimate, but with 95RON UL + Millers, you know the highest octane you're ever gonna achieve is ~97RON...

As stated, if in doubt, get a Knocklink. I've experience a couple of Christmas trees using 95RON UL (Texaco). It concentrates the mind, believe me...