Blood test for drug driving - England

Can a re test of sample A be requested in England ? Do the police have to say sample B should be tested within 5 days and do they need to provide booklet where to get tests done? Different solicitors say different things.

Anyone know decent solicitors or Barristers that are reasonable price and won't rip you of.

If you have mental health problems and want to try represent yourself, with some help of a layperson ....how is this viewed?