Just a quick update for you. Thanks for all your help, I'm all finished now. It was much easier than I expected getting everything back again. The only real problem I had was with connecting the manifold to the cat pipe but got there in the end. It was easier to remove the manifold studs before connecting the cat pipe, but once this connection was done everything just glided back into place

One thing that occurred to me though, was, the first time round I inserted all the manifold studs and then put on tthe gasket and manifold. When I found that I could not get the front pipe onto the manifold studs would it have been easy to just onbolt the front pipe at the coupling? I didn't do this at the time bacsue I thought it might have been secured with exhaust paste - but now, reading the haynes manual it does not imply that this would be the nprmal case...
On the MPG front, I noted the following:-
I drove from Chesterfield to Swindon which is 153 miles. It is 4 miles to the M1 from my house and I reset the mpg counter as soon as I got on the m-way, and put it on cruise as 75mph. I should say, that I have a new speedo and it is exactly accurate according to the sat-nav, whereas my last speedo showed 3-5mph over.
The mpg settled at around 36. After about 8 miles there is a 6 mile section of roadworks at 50mph, so I set the cruise to 50. By the end of this stretch the mpg was at 40.2.
I got bored on crusie so drove the car from that point between 75-85 but I tended to back off back to 75 when I realised the speed had crept up. When I got off the M5 the mpg was 39.8. I had an uphill stetch on a single carridge way for about 2 miles and after this the mpg had dropped to 38.3. After about 20 minutes along country lanes stuck behind cars doing 40 I got to my destination with 38.1 showing.
Over the next couple of days I drove around Swindon (about 10 miles worth) and the mpg dropped to 35.2, then 33.6

I drove back home yesterday in the same style as before and starting from 33.6 managed to get it back to 37.4 when I reached home.
I don't know how significant this is but I hade the vehicle geometry set up by a company called Wheels In Motion in Chesham. They used a Haykeye machine. There is one adjustment (I don't remember which) that they set differently to the manufacturers spec because they say the setting is improved, so possible that this makes a big difference to ecconomy. Have just noticed though that front wheels are scubbed on insides... going back next week for them to check.
Cheers
Chris