I brought a few months supply from my local Sainsbury as they were only 25 pence each as they were close to their use by date. Didn't notice any difference at all and the last ones were around 4 months out of date.
Gideon I did the half. I had hardly ran in the 4 weeks leading up to Ealing due to a cold and then food poisoning and promptly got another cold after it so ended up running just a handful of times in a 6 week period. Stevenage was so wet I'm glad I bailed on the full!
I loved it this year. And yes it is steep but it;s worth it. I run several halves a year and some for as little as a tenner. It all averages out over the year and it's nice to do a couple of big closed road events of which there aren;t many in London
For anybody relying on public transport Saturday events are a godsend. It's also nice to have some weekend left after running. But I can fully understand why organisationally Sunday's are often more practical