There are not many cases where the results fail to come back or time out after the 6 months to raise a summons.
there are cases when the result comes back under but the police (cant be bothered) forget to inform the suspect but if it is over you are almost certain to hear.
There is not a lot that a solicitor can do pending the results, that you cannot do for yourself.
You should have been given a part of the urine sample to take with you. You could have this analysed yourself at a laboratory to compare against the police sample. That will give you peace of mind that your sample WAS the one that the police had analysed in your name as they should match up.
You can request the police to supply you with a copy of your custody video. That can be used to show if correct procedure was followed. It can be good to apply for it early rather than leave it 3-4 months until you get the urine result as some forces recycle the videos after a few weeks.
The two scenarios that are most likely to produce errors in procedure (but not as often as specialist solicitors make out) are urine samples and hospital procedure.
One other thing to consider, to be honest it does not seem that you are concerned about the result, more that you are looking for a loophole to get out of being over the limit....... if that IS the case, you might NOT want to compare the result of your sample against the police one as that would tend to help the prosecution case if this came out.
Solicitors will always be optimistic, but remember they never lose as they get paid whatever the result! To mount a technical defence, challenging the chain of custody, is likely to cost you anywhere between £3,000-£10,000.
If you are successful, you will not get your all your costs back. The best you could expect is to be awarded Legal Aid rates of pay which would only cover perhaps £500-£1,000.
If you lose, you have lost the discount on the fine for an early guilty plea, the court costs will have gone up from £85 (pleading guilty) to at least £650.
These are the hidden risks that solicitors conveniently ‘forget’ to mention.