1st post - I have seen old threads on this issue. I spent an unhappy hour (in the dark!) trying to refit the tank on my '06 1050 ST last night, after changing the battery. It took an unhealthy amount of swearing, shoving, swearing & forcing to get it back on the bike. It clearly wasn't like that when it left the factory & I put it down to not having any workshop/garage space or time to do maintenance to the standard I'd like. It makes far more sense now (though is, if anything more frustrating) that I find out there is a known issue with the fuel tank distorting. It has clearly been rubbing around the headstock & there is a mark where the left bar has made contact with the tank at full lock.
Anybody know of any solutions?

Check out the Sprint section at triumphrat.net. This is a known problem with the 1050's, both ST and GT. The inner tank is plastic, and can change shape if off the bike too long, making remounting the tank difficult. There are threads on the subject where people have shared their tricks.