Thanks to drj69uk, Nev, XPCobra and many more of you out there who have contributed to discussions in the past and helped me recently.
How I've spent the last few days:
New manifold, EGR valve, actuator, timing belt, roller, tensioner, waterpump, auxiliary belt, thermostat, coolant and vacuum pipe...
and a couple of weeks ago:
New strut top bearings, front discs and pads.
The IS3 Signum is going better than ever. I always went on about how much torqueier my Golf TDI 150 was - well the Signum is every bit as impressive, probably more so because it does it all so smoothly and with a sense of total road domination - brilliant!
The biggest surprise was the effect of 59p worth of vacuum hose, having changed this I now know that there must have been a leak here ever since I bought the car 2 and a half years ago - that's what was making the car feel soft at lower revs. I'd recommend anyone with a 19DTH engine to have a close look at this for cracks or perishing, even if you feel your car drives fine - you could be missing out on some tremendous performance for pennies and five minutes!
The thermostat likewise has never been right, a bit of a pig to fit but the needle now sits between 80 and 90, still a bit all over the place though, but my heater is much improved and the engine's probably happier running at the proper operating temperature.
By the way, I wouldn't recommend the manifold job to anyone over the age of 40! From my experience, it's far too much of what's left of my life to spend on one job - but then I'm pretty slow, careful but SLOW.
Many thanks again to all my fellow Vectra C and Signum owners. You made it all possible and saved me a fortune. Happy New Year!