So just cancelled during my free trial, and it seems to have gone okay. The initial call queue wasn't too bad - sometimes with these products where you have to phone to cancel you always get "due to unexpected call volumes", so was pleasantly surprised I got through in under a minute.
To anyone cancelling, be firm and say you want to cancel, be polite too as the call centre person is just doing their job and probably have to go through a script

. They will offer you additional free trial alerts, reiterating no cost but then ask to confirm last digits of your card - you need to again say you don't want the trial (unless you really do want to try it??) as I guess they are hoping that because its free people will take it up!
If you need a report and eligible for the free trial, then don't be afraid to try it. Cancelling is a bit of a hassle as you have to phone up, but so long as you are aware they will offer a 6-month alert trial that you need to turn down, then its simple enough.
Of course if you are not eligible, a £2 standard report is cheaper, unless you need your score as well as report, even though only you see the score not any lenders.