I have fixed ours a few times - Usually, I find the problem is that the springs break. You can either reform a loop of the existing spring (not easy) or this ebay item 112006680250 (Tension spring 2mm x 12.9mm x 138mm) might work as a replacement - Bought some but not tried yet as the previous temp fix is still holding (2 years).
To access it you need to take the oven out and remove the side panels (easy).
Rob
January 2017