Hello, thank you for your question. My name is***** am a barrister and will assist you with this issue today.
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In terms of there being "traces of cannabis" unfortunately for you just traces can be enough to put you over the limit. The limit is very low in the UK.
If you plead guilty or are found guilty you can expect a driving ban and a fine if the level of cannabis is just a trace.
In term of gas poisoning I doubt that would affect the reading but you would have to speak to a forensic expert about that. If you use cannabis anyway I'd say it is more likely that it is the cannabis that caused the reading rather than the gas.
If you are going to plead guilty at the first hearing then you will likely be sentenced there and then. You don't particularly need a solicitor to attend with you if this is the case as you can explain everything yourself and put forward your mitigation about needing pain relief etc. If you do plead guilty at the first opportunity you will receive 1/3 off any fine and lower court costs.
If you plan to plead not guilty or you want to explore the issue of the gas poisoning I'd suggest fully instructing a solicitor. You can find a local criminal solicitor here: https://solicitors.lawsociety.org.uk.
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