The oven door seal is sagging, and lets out a lot of wasted heat, are they easy to replace?
The first thing to check is if it is a 3 or 4 sided silicone seal. If so, these are quite easy to replace. It is also a good idea to check that the door latch is in good condition, otherwise if it has gone weak with age (as a lot do), then changing the seal will only fix PART of the problem, if the door does not close snugly against the seal. The UNIVERSAL seal kits can be obtained from between 7 and 15 depending on the site, but specific model spares will be about 20. There is a UNIVERSAL DOOR LATCH available too, for about 5. You may be lucky as a lot of manufacturers use slotted holes for these to take different stock spares. Once the inner and outer door is unscrewed, and taken apart, it is obvious how to change this part. Click HERE to see a HINT article on seals, and sites that stock the kits.