Well, the large tank finally arrived from Clarke Manufacturing in Oregon. There was quite a delay as it was held up in UK customs.
I fitted it in 40 minutes today and took the bike out for a quick ride this evening.
Fitting the tank was fairly straightforward, Clarke advise that getting the seat on after fitting the tank was going to be difficult but it was not bad.
The tank feels very wide - I guess I'll get used to it, after all I will be getting used to not running out of gas so easily.