power is cutting out on my (electric) boiler

I have an electrim mashing bin (the standard one of these) that I use as a boiler. This morning as I went to heat my liquor, the element started to cut out (water was around 60°C). When I left the power off for a while, the element turned back on, but then cut back out after a couple minutes. I tried this a couple more times then gave up on brewing.

I figured the problem was scale build up on the element causing overheating (I live and brew with London's rather hard water), so I used some spirit vinegar (about 150ml in 7L of water, plus or minus) to descale. It had a visible effect, removing a decent amount of scale, but the element still cut out after a few minutes. Fast forward a few hours and I repeated the descaling process, at a higher dilution, about 500ml of vinegar in the same amount of water. Alas, the element still does not seem to stay on, though it did stay on for nearly 7 minutes just after the most recent descale, now it won't engage the element at all. (element now appears very clean and generally free of scale, I can attach a photo is anyone thinks it'll help.)

Any assistance at all is very welcome.

edit: Here are a couple shots of my boiler: top down of element and manifold:

side view of thermostat (currently disconnected from the element)