I took car in for an oil leak, have a bill for £2350!

The problem with a oil leak is where it is. Often it can require disassembling a large part of the engine to get to it. Open the hood and see all the wires and cables etc.

I suspect yours was more of a valve cover leak which is not the worst (Some leaks you have to take the transmission out...... your vehicle is 4WD or AWD and has driveshafts front and rear...) but can still be timely.

In the USA, there are standard rate books. You basically look up the job, it tells you how many hours - multiply that by the shop rate and you will know exactly what your bill is. If the mechanic is faster, he makes a bit more money. If he is slower, he looses.

Note, if the job requires ... removing spark plugs and the mechanic re-installs new ones. Typically you only pay for parts for incidental work. Often that is like changing the fan belt etc. On some cars, when you change the timing belt, they recommend you go ahead and change the water pump. You pay for the water pump job and add the cost of the timing belt part.

Warranty work is the worst. The manufacturer assumes the mechanic will learn short cuts (maybe he doesn't have to remove a part, just loosen it to get it out of the way) which the mechanic will learn after doing the exact same job 10-20 times. But those 1st couple of times is almost a guaranteed looser for him.