Its an mg tf convertible, how much will this cost approx

A lot or a little... depending a lot of information you failed to provide. I know of labor charges that vary from $25 an hour to over $175 an hour. A lot depends on where you live... who you have do the work... and any other problems that will or should be done at the same time.

MG Tf has a very simple 4 cyl pushrod engine, all cast Iron, maybe 11-12 head bolts, 3-4 manifold bolts, tilt back the SU carbs, pull the rocker arm assembly, drain the water, undo the head bolts and it's off. Actually a pretty straight forward DIY job, at least for me, I've done a bunch of MG, TR, etc. Head off in an hour. Real cost may be in any repair/replacement if the head is cracked. If this were my engine, I'd do a valve job too. I'd budget 8-10 hrs labor plus some time to have the head planned, if needed, or at least checked.