If SJSC gave you the sizing, then stick with it, they are very good at sizing people up. (See their website on bike sizing)
As you are going to spend a fair few quid on a decent bike, it makes sense to get one of the correct size, as it will last you years if looked after.
Whilst a frame 1 size to small can be made 'bigger' by putting up the saddle and getting a longer handlebar post, likewise a frame 1 size to big can be made smaller by lowering the saddle (as long as it's not on the lowest point, flush with the frame) you also will probably need to repace the handlebar post to make it shorter (so the bars are closer to the saddle). The critical thing with a frame that is too large, with your feet on the ground, you should have at least 1" clearence between the bike and your wedding tackle, otherwise you will be joining the Vienna Boys Choir the first time you need to do a sudden stop.