It won't be £300-400. Start thinking more like twice that. there is a bit more to the JTS than a TS. Also, if the head is off, you have the chance to do the obligatory decoke to inlet valves and ports. There should be no subsequent damage.
Parts cost will be about the initial hoped for cost.
Cylinder head gasket set £120
Timing belt kit £150
Engineering shop costs £100
Labour- say 10 hours £500
Consumables & fluids £50
When I looked at mine, it became obvious the valves needed to be removed before skimming the head. Prices have gone up for this sort of thing in the past couple of decades and newer cars will easily swallow £1500 in many cases (even for 1.6 TDCI/HDI engines as fitted to Peugeot, Citroen, Ford, Mazda & Volvo).