Thanks for advice! I'm thinking of taking to another mechanic to see what they say and my mechanic is saying he thinks it's most likely master cylinder?

It's either air still in the system or the used caliper is faulty, ie whatever adjusting mechanism is in use (if any) isn't working, or the caliper isn't the right one for the vehicle...ie rear caliper designed for vented discs now operating on solid (narrower) discs or some other issue like this.

For a normal car like this, no way would i recommend anyone fit used brake parts, the breaker car might well have been written off due to poor brakes, properly reconditioned or even new brake calipers are fairly cheap for common vehicles anyway for the usual sources.

The mechanic is the bloke to ask, he can't get a solid pedal it would appear after bleeding the system three times, he's seen the thing and worked on it, what does he suggest is wrong?

On assumes he bled the whole system, not just the caliper he replaced.

If the flexi pipe has been clamped off then there isn't any need to bleed whole system, unless it wasn't clamped off properly which often happens, but sounds like the calliper is faulty which comes as no surprise from a second hand part.

which is a surprise a mechanic would use second hand brake parts?