I have just found scan of the marriage record. It looks like George has made his mark, there is a faint X by his name and the handwriting is the same as in the main part of the record. Sarah has signed her name and so have both witnesses. As they were married in a church some way from where they lived and the groom was illiterate, I wouldn't worry too much about a slight mis-spelling of his middle name. As Sarah would be under age and George possibly as well, maybe the distant wedding venue was to get round lack of parental consent.
Another idea, if Catherine married, you could compare her signature as witness with the one on her own marriage record.
Added:
There are banns between Catharine Parrish and John Morris (her name spelt exactly as she signed on George's marriage record) read in St Mary Whitechapel, late Jan to early Feb 1828. Marriage in Chingford on 25th May, only a transcript on Ancestry no image. I think I have found the couple on 1851 census, Catharine born Waltham Abbey, so looks as if it is the sister.