OK; I dare say you could fly a published "RNAV" approach using any RNAV RNP0.3 nav system, which could (at a stretch) be an FMS driven off an INS which uses DME/DME corrections....

That's because the published plates are entitled "RNAV/GNSS" or similar. One doesn't actually have to fly them with a GPS :)

Actually, im EU based and fly according to EU-OPS. Still legal to do without GPS.

But yes, for the private flyer (outside of those rich enough to afford their own jet) GPS is the only available means currently to fly those approaches.

Not quite - see para 6.1 of AD1-1 in the UK AIP. In short, details of PEC for your aircraft should be in the AFM/POH. In the absence of such data then 50' has been found to be suitable for a wide range of light aircraft. A good example of when not to add 50' is the humble Grumman AA5, where the POH specifies a PEC of 15'. Don't be put off from fighting your corner when you're right ;)

That's useful - I didn't know that.

My POH shows no PEC (that I can find). I thought the POH does not have to show a PEC if it is below 50ft.

So for the IRT you are supposed to add 50ft (if the POH shows nothing), or the POH PEC (if the POH contains one).

I think adding 50ft is a whole lot simpler than fighting a corner, regardless of whether I am right :)

Correct, if no PEC shown then add 50'. If a PEC is shown then use that. Some of the EFIS aircraft I fly are certified as precision altimeters so we add nothing - makes asymmetric go-arounds from a DH of 200' with screens up a good test of flying accuracy. And as it should be done of course. Sadly I'm not very good at giving in when I know I'm right :E