Update.
Now have this car sat on my drive. Have done a little bit of investigation and found the symptoms to be a little different to the original diagnosis.
No sign of mayo on the dipstcik.Oil looks normal to me.Have flushed the cooling system three times. Got lots of creamy sludge out but still a lot more to come by the look of it. I can only assume the garage wasnt aware of how much flushing would be required after oil cooler failure on V6. There was no heat from the vents,but after flushing now have hot air which is hopefully a sign of progress. Havent done a compression test as yet as I dont have a tester,but I may get one next week. Havent tested the old cooler as I dont have the means (or know of a suitable method) of doing so.
No sign of overheating either,quite the reverse in fact. Temp gauge wont move off the bottom of the scale on tickover. If it is revved for a minute or two it will creep up to 80 degrees,so it looks like it needs a stat.
Engine sounds a bit burbly (moreso when cold) but there seems to be a bit of an exhaust leak, which could be the cause.
Im going to carry on assuming it just needs plenty of flushing,but will also look at other things to try to prove myself wrong,if that makes sense.