Ian,
Like Derek, where possible, I buy locally at discount.
The sydicate buying mentioned by Dave is a good way of saving but requres trusted associates, sufficiently local to each other to avoid too much cost and time in re-distribution.
Especially where not available locally, I tend to buy in fairly large quantities. This has to some extent back-fired on me lately - the use of micro-splitters on the majority of jobs means that many of the 'conventional' chemicals will sit on the shelves of my stockroom much longer than I antisipated. Some may out-stay their shelf life and be discarded.
With regard to the rising fuel costs, the accounts software I wrote many years ago, asks for my current mileage every time I enter a fuel invoice ( I always fill the tank). It then presents a report displaying details such as MPG and fuel cost per mile.
This last figure is very telling - if it jumps up a penny it takes no great effort to work out the the cost of a 10-mile round trip has increased by £1.
John.