TDi 300 coolant loss

Hi
Last year my Discovery TDi300 (175k miles) suffered a water pump failure which caused a major overheat & blown headgasket.

In repairing I had the head skimmed true (needed quite a lot) - but not the block which remained in situe. (obviously new gaskets etc etc )

Subsequently - the car now uses coolant (no obvious leak) and has to be regularly topped up to avoid a new overheat. Otherwise she starts & runs fine.

A LR dealership has advised that tdi300 heads cannot be skimmed (as they are case hardened ?) - and that if they are - often coolant usage results.
Is this true ?

if so then I guess a new head is the logical route - but is there any way to diagnose & confirm head at fault rather than (say) a cracked block ?

Finally - does use of this sort of product actually work
http://www.heal-a-seal.com/TECHNICALINFO.html
(assuming that I have reduced the integrity of my engine by skimming) ? - anyone had any experience? or is it fools gold that causes more harm than good ?

All advice/experience appreciated.

Phil