I had the same problem with my Whirlpool oven as the post below. I
replaced the thermostat at a cost of 35 UKP from
keepingupappliances.com. The thermostat is connected into the rear of
the oven. An easy change removing about 10 screws. Took about half an
hour. Thought I would post this as I couln't find any followup answer
to the original thread. All working well now. Wife is off my back...

We were wondering if anyone could tell us how hard it would be to
replace a thermostat on a Whirlpool AKP510 electric oven, more
importantly where the thermostat is located. The problem with the
oven
is that it doesn't heat up. When we switch it on at 250F the power
light comes on but if we turn it down to 220F the power light goes
out, even if we leave it running at 250F it doesn;t heat up properly.
We have sourced a spare themostat from CPC for 30 pounds.

I replaced a thermostate but it was a small circular thing on the top of my oven, was the one you replaced a long tube attached to the back of the oven??