This isn't just a boiler...

[Scooby]

Many of us on here have used the cheap kettle elements in plastic buckets to make very inexpensive boilers. That bucket is polypropylene which can stand boiling wort easily but does get more flexible when heated. After much use the surface stains a bit but will last for years.

It is worth making at home but you will need to cut a hole for each element and a hole for a tap. Holesaws are best for the job but if you have a steady hand and take it slowly a stanley knife with a new blade will do the job. Hope that helps

[Subsonic]

The white crud connects a bimetalic strip to a thermal cutout. If it cuts out remove the metal circular bit with the crud on and the plastic pin behind and all will be ok.

[anomalous_result]

Mine's been fine yeah. As has been said it does get a bit flexible when hot, I wouldn't move it once boiled, but if you plan ahead it's fine.

I would rather a metal pot though, it's just more sturdy/durable.

Also I don't think M&S kettles are a fiver anymore.