Please can anyone suggest a fix or advise, I have noticed over tbe weekend that the rubber gasket or seal had come away in the centre,  is there a way to fix this?

Has it split where it joins the bodywork, or has it just pulled out.

Hi, it's not split just come away. 

Drop in to a local windscreen repairers, they will likely use some windscreen mastic and push it back in place. Had the same with my previous Honda Civic, bit of mastic, job done.

I had the same problem. First time dealer glued it back, then it happened again. Second time they replaced the glass (with the worst glass!) and still needed to shorten the rubber so it doesnt rub the roof line when the trunk is opened. I suggest regluing and shortening the rubber.