You don't need a special mast, but one that's strong enough for the windage. For an X50, that'll be pretty light, so almost anything will do.
Aluminium tube is the mast of choice, strong, light and flexible, although steel will work just as well provided it's not too long (more than about ten feet). Aluminium scaffold tube is the king in my book but it's hard to find now, being illegal. Steel scaffold will do, but needs to be very securely mounted.
Heavy "T & K" brackets can be found at most TV/HiFi shops, or, as you say B&Q etc (maybe). Expect to pay around 20 quid for the brackets, another 10 for some nice big self expanding bolts.
You can expect a local firm to fit everything up for about 150 Pounds if you don't want to shimmy up there yourself.
Good luck with the test next month!