The garage is required to complete the work within a 'reasonable timeframe'. But the 2015 Consumer Rights Act doesn't actually stipulate what length of time is 'reasonable' and which isn't.

I would advise that you contact the dealer and try to agree on a deadline for the work. As a minimum, I'd expect them to supply a loan car. If they don't, make a record of all receipts and costs you have incurred while your car has been in for repair - you'll need these if you want to take legal action at a later date.

If you are unable to negotiate a loan car or an acceptable deadline for the repair then you may need to get your car back and pay for another garage to do the work. You can then make a claim for the time wasted/extra costs incurred via the trade organisations the dealer is a member. Or, as a last resort, seek legal advice for a claim via the Small Claims Court.