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Help repair our ancient oven!

We have a Shacklock Custom 27 Wall Oven (Jumbo) that apparently hasn't been functioning for many years. The timer is jammed, and the fuse looks okay, so we think the timer has something to do with it not turning on. It has gas hobs on the top and these work fine. We would love some help with this, as we are unsure if it is worth repairing/an easy fix, or going to be too costly.