Thanks, should be 300gsm if I remember correctly, is only really visible on the top and bottom of the print.

I presume there is a back board and dust cover but is the back of the frame touching the wall or do you have spacers?

Yeah, there is a backboard and dust cover, no spacers though. I will add tonight, although it's probably ten years too late.

Yup - it's atmospheric humidity (I was informed by a pro art restorer). I've got a professionally framed Donwood print on heavy paper that has suffered the same. I live in Brittany - I think it's the French capital of humid... I suspect Scotland's pretty similar! I've not had any luck getting rid of it and jusy accept it as it is now. If you find a fix let us know!

Actually the over mat usually keeps the print from rippling better than a float. Paper changes sizes when the humidity or weather changes. Could be the season now where you live the humidity is a bit higher and unless the print is in a well sealed frame that is what causes the shape to change. It will return to it's original shape once the humidity changes. That looks like a BAST Mickey...I'm not sure what sort of paper those were printed on, but BAST's prints in particular are not really affected in value by their condition. He's pretty rough on his works on paper. I had the privilege of visiting his studio on a few occasions and when he brought out works on paper for viewing he would scatter them all over the floor and walk over them.

When a print is hinged for an over mat ideally they use two "tape" hinges so the paper can move as it changes shape. If they "taped" the print improperly so it can shift and move that might cause rippling. You might check to see that it was hinged properly.