N53 specialist required NE London or surrounding

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I bought an n53 engined e60 525i and have been used to a bit of a rough idle from cold for the past few months. No codes were being thrown up.

A normal CEL has just appeared and a half filled CEL is now flashing beneath it in the bigger diaplay and the car did not want to start but has just soldiered home on reduced power, about 5 miles.

Reading around suggests anything from coils to injectors and HPFP. Unfortunately I don't have too much time to hunt this down, so can anyone recommend a specialist near North East London that preferably won't pull my pants down!?

Thanks
Re