From Homebase of all places

Apparently, it was going cheap. (it wasn't difficult to see why)
It is 32'' x 24'' in a flimsy black flat thin frame with an MDF backboard. Close framed and not hinged to the backboard.
I think the reason it was going cheap was that the picture was wrinkled along the top and down the RH edge.
When she bought it I knew absolutely nothing about framing and suggested that the wrinkles may come out, if it was hung in the dining room in a warmer atmosphere than it it had probably been kept in at Homebase.
Twelve months down the line the (what I now know to be

This morning I took it out of the frame. (it actually looks much better out of the frame


The problem is not atmospheric - it is that the print is too large for the frame. The MDF has a 2mm gap.
The print is 2mm oversize.

The print is on gloss paper, I would imagine at least 200g/M thickness. I can easily trim it.
Q: If I lie it flat on my bench, in a domestic atmosphere, will the cockles come out of it?
If not, is there any way to encourage the cockles to disappear?
I'm confident someone will have the answer, so thanks in advance.
