Hi RiverSkipper. A320 seat plans at Gatwick are a bit of a lottery at the moment.

Seatplan 1
The standard Heathrow A320 seatmap is this one (all seat maps from BA Source) and this is the one in your post, ending at row 28:
http://www.thebasource.com/seatmaps/...8pinnacle.html
These aircraft have been loaned to the Gatwick fleet in the past.

Seatplan 2
The ex-bmi A320s, which were operating out of Heathrow but have recently switched to Gatwick have this seatmap, ending in row 29 but with no row 13:
http://www.thebasource.com/seatmaps/a320/cy168.html

Seatplan 3
The second-hand A320s operating from Gatwick have this seatmap, ending in row 29, with a row 13 but no 1DEF seats:
http://www.thebasource.com/seatmaps/a320/cy171.html
These aircraft have the overwing exit rows at rows 12 and 13, unlike the others which have the overwing exits at rows 11 and 12.

Seatplan 4
The seatplan in your MMB is a fourth seatplan not shown yet on ba source, a variation of Seatplan 3. An extra row of seats is being fitted to the second-hand Gatwick fleet. The seats replace cupboards not shown on the current seatplan on ba source so there is apparently no change to the legroom.

Your actual seatplan could be any one of the 4 above hence c-w-s's sound advice not to take the MMB seatplan too seriously.