Seems right, if the pitch is being relaid I don't see an issue with them hosting this the day after our game.

Rod Stewart on the 14th June

dunno, but it's not full length

The Olympic stadium wasn't 100m? Don't think it makes a difference only the gap between 30 to 40 metre changes and ingoal narrowed

Granted this is a bit sad, but, being the inquisitive type. I used Google Earth Pro, and the measuring tool to come up with the following. The pitch is 68 metres wide, 105 metres long DBL to DBL, and GL to GL it is 94 metres. giving a 5.5metre in goal.

This means it's a total of 7 metres short, the in goal areas being half a metre short, and the pitch 6 metres short of the recommended length.

Approximately

When do have to give the posts back to the Primary school and how much are some adult size ones ones

Headingley's pitch lasted nearly 50 years, but it's things like all year round rugby/football/events, changing weather patterns and stadium designs that aren't conducive to allow the pitch to grow/develop as it should.
For instance at Headingley the Carnegie Stand reduced the light on to the pitch. Stadiums used to be more open with open ends rather than all covered. Not to mention, that they always got the summer to recover from the hammering they got in the winter. Of course they don't get that now.
Also, if we remember back there were some god-awful pitches back in the "good old days" that we just wouldn't accept playing on now. Headingley used to be a mud-bath by mid-season.