In the close ups of the shrunken Martin in the goblet, the background is the same when Tim is standing up and when Tim is sitting in a chair. With two different situations, the background should be different.
The year 1215 was only 49 years after the conquest of England by William of Normandy. At this time, the common people still spoke Anglo-Saxon aka Old English, and the nobility spoke Norman French. Without fluency in these languages, Martin & Tim's Modern English would have been incomprehensible to the people they met in the past and their speech would have been just as opaque to the time travellers.