Why did MYT’s 767s leave for a year and also howcome they didn’t seek newer 757 as older 757 replacements.

The 763s left to do work for Skyservice over in Canada, where they were more needed than they were with MyTravel, however with the fleet reductions that have happened at MYT, they are needed again.

The 757-200s that were disposed of were very old, and at the time it was believed that MyTravel were moving towards an all-Airbus fleet (which they probably still are), but they kept G-WJAN for the capacity, and also it being the youngest of the lot (with MYT disposing of those 757s with numbers in the sigle-figure range).
It seems the capacity provided by WJAN isnt needed anymore, so it will return to Skyservice and will probably not return.