What is the difference between these two (I have manuals for both) and can I use them?  If so, how?  At the moment the DTV button brings up sky.  I appreciate, therefore, that I don't actually need them but out of curiosity, the info would be useful!

Depends which you have built in.  I think HD is fairly standard in the UK, but you could opt for the older one up to a point.  Unfortunately, they'd never let you save the cash and have neither!

If you've got Sky, it's possible they've disabled it.  The only use I've found for it is to wake up to radio - I'd place more faith in the DVB tuner than my sky box.  And the play timer doesn't seem to recognise the new 4 digit 0xxx channel numbers for radio anyway.

If DTV is bringing up SKY then the DVB option is turned off.  

There are some uses to turning it back on, and turning off the (analogue) TV option instead, allowing you to allocate Sky to the TV button.

If you are ever recording two programmes off of SKY (which my wife frequently does), then you can switch to the in-built digital terrestrial tuner via the DTV button (once you have turned it back on in the Tuner Options MENU).

DVB-HD - my understanding is - is a bit misleading in the UK, it is a Euro only benefit and will require both hardware and software changes to work in the UK - best choose it without this option if purchasing new and buy a decent FREESAT HD PVR!