Should I ask for my money back - crack in Titanium frame
As you're in the UK, the Consumer Rights Act applies. It would seem that a £5,500 bike that lasts less than two years isn't of "satisfactory quality", but I would be surprised if the shop simply gave you a refund. I think you have to give them at least one attempt at repairing the frame (I don't know how feasible that is for titanium), and take it from there. If you can, I'd talk to the Citizens Advice Bureau and/or Trading Standards if the shop gets difficult. All this will be easier if you have proof of purchase, of course. 

If I were you, I would want a refund, but would settle for a high-quality repair (and if it failed again in less than several years, I'd want the cash back).