LOW MPG ON SWITCH TO BIO DIESEL.

Hi all,
a local guy has been selling bio diesel for about 8 months so i thought i would give it a go as all the taxis around are on it with no probs. he told me to run 50/50 on the first tank then run on neat bio. My 20 year old 300 gd and an astra 1.7 td are both on it however both are experiencing a massive decrease in mpg. I used to get about 500 miles on a tank full in the astra, will only probably manage 350, my wife uses the g and usually uses about £20 worth a week, this is now at about £40 per week so you can imagine the flack i am getting. Is this normal/ he sells it for 85p a litre so on balance is pretty much the same performance per pound, any ideas or am i missing something.