£375+ vat.....that's a bit rich! I did mine myself, cost was £4.75(tin of lubricant) it took me a couple of days, but I had all sorts of problems down to stuck pressure relief valve and a stubborn motor. If I was to do it again I reckon 1hr including sorting out the motor and valve. If that company has done a few they are really taking the mick on price. Are they fitting a new motor? They are £40 from AliExpress but there really isn't any need to replace the motor, they get so little use the rotor and brushes have barely, if any wear. A soak in diesel, squirt with wd40 or similar, light tap with a hammer to free the brushes, connect a 12v supply and it will burst into life. A fair price would be £150+ vat and no need for a motor. Why not try Durham Dave on the Z4 forum, he does them in under an hour and charges £80.