Well, got a little carried away, took the Zorki, Canonet QL17 and the Mamiya Super 23.
Zorki 4K got loaded with Ilford FP4, took one shot then went inside, so that was a bit of a waste.
Canonet was already loaded with HP5, this held me in good stead as a back up shooter for the fast stuff I just wasn't good enough to get with the mamiya, and produced the most consistent pictures. Was also handy as a backup shooter in the reception, once mounted with a small flash.
Mamiya was excellent, but very bulky and stressful when it came to remembering to get all the settings right quickly. However took some excellent shots that I am enlarging to 11x14 (ish).

Things I learnt. Definitely carry a small camera along with the MF. Don't carry to many film backs all loaded with different film - HP5 in MF actualy looks fantastic blown up, almost better than the PanF 50 (though I am a street photographer with natural bias!). Next time I would have two types of film, not three, and would have several backs loaded with both ready to go. Don't have a 6x7 back in amongst 6x9 backs as you will forget..! Don't get in trouble with the missus when you get invited by the pro to come along to the photo shoot off site as you are supposed to be a wedding guest not the pro's off sider... still the missus came as she new the layout of the location better than any one else including the pro (whoops!)

Thanks for all the advice guys, think I would love to do it again, but with less stress and a bit more organisation. Still I definitely got some shots the pro didn't, so once mounted I'll have a nice gift for my Bro and Sis in law.
Dan

P.s fill flash kicks Ass, but is another calculation I had to do whilst light metering setting manual speed, aperture, and remembering to winding on the film, changing back, put back in the dark side, pull out the dark side and focusing ... Damn those old pros must have been good.