Cheers Mikey, you are of course telling me what my brain is still trying to avoid

There's a great step by step guide on the Disco forum which even I could manage as long as its done nice and steady.
Valves on fleabay are showing as £420 a set from Maltings, or £160 each from London both claiming to be genuine parts.
I believe I need gaskets as well.

Any other advice?

£420 is borderline cost price for a pair of valves. There are also 3 gaskets per side needed to fit

Attended the Alive tuning agent as arranged, couldn't find their unit at first and they graciously sent a chap out to find and escort me in. Landrovers all over the street and clearly a busy place despite its backstreet run down appearance. £54 lighter for a diagnostic and told the same as I already knew, never mind the £30 in diesel getting there and back. To be fair they confirmed it was only one EGR on the drivers side and that the hoses were all spot on. For £550 they could fit a replacement if I came back another day next week. Strangely they wouldn't give me a quotation and just scribbled some prices down on paper but wouldn't let me have that either.
After a two hour wait they confirmed my "European engine was not suitable for blanking and remapping". Wish they had told me that last Saturday instead of all the messing about. They had quoted £274 for an EGR valve but I picked one up today at Malting's in Doncaster for £225 including gaskets which I intend to fit this weekend. Car is now in constant limp mode of 30mph and after 70miles additional motorway journey today in the slow lane being overtaken by forty footers it looks like I'll be waving this old girl on to pastures new. Certainly would have another rover but newer with less miles.

I'll let you know how the fit goes in case its useful to anyone.

EGR fitted by myself using a very good step by step guide. Technically it wasn't that complex just very fiddly in parts but I'd certainly do it again. The EGR was blocked so much it stuck open. The inside of pretty much every hose and pipe on the drivers side was caked in soot. I scrubbed what I could and used some carbon cleaner spray on the MAP and MAF sensors.

Well everything fired up straight away which was good, the EML light disappeared but the Restricted Triangle stayed on. Lots of carbon smoke poured out but cleared quickly and I guess the system will be clogged for a while yet. Filled the tank and took it up the A1 from Leeds to York, Restricted Triangle stayed on but performance was fantastic, acceleration great.
After an hour walk around we headed home and suddenly I realised the Restricted Triangle had gone. No dash lights, excellent performance-amazing. Sadly after ten minutes all hell broke loose- traction control warning, emergency brake failure, restricted triangle back. Stopped reset and all fine until I accelerated to 50mph then dash lights again.

So that's were I stand, drives fantastic unless I accelerate to fifty, I'm guessing its still clogged a bit. Any ideas?

The offending EGR (grrrr)
Exactly as I found it, stuck open. The devastation it has caused is unbelievable.

I remember having to clean or blank off several EGRs on Rover 75s and MG ZTs, which gave owners an additional boost of extra power. Unfortunately the newer cars, this cannot be done to, which is a shame, the cars mentioned all passed every MOT, so all the efforts to make us greener, just cost more on purchasing new cars, and cost in extra fuel/performance. I think crazy world now, with all the extra things fitted, can't chop out the cats any more now on modern cars, although on previous mentioned, made no difference at passing MOTs, but technically should have.

Hope you can get yours sorted, but if the pipes and EGR are still clogged, they would quickly interfere with signals to MAF sensor, causing more problems, this use to be a regular reason to replace through lack of power etc, try unplugging to see if it improves, you will get smoke and use more fuel, but can indicate if knackered.  John

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Checked the rear air hoses at the back of the engine top (the short ones) and noticed they didn't seem to be centred in their spacing. Closer inspection showed an obvious air gap and with embarrassment I re-sited them then tightened them up. Could have slapped myself stupid as I then drove effortlessly up the motorway with a clear dash.

Note to self- Check, check, then check again!

Thank you everyone for all your help and advice.