Thank you for your replies.
Firstly I should state I am in the UK.
Neilestrick, yes I do mean power cord and the same type of cord has been used as the original. Kiln is the model HT45 with a rating of 3-3.5kw, 230v, 13 amp. I use a H3i controller with 9 programmes.
The problem seems to be with the bottom element. I have bought 2 full sets in less than a year and used the kiln on average 2 or 3 times a month. Glaze firings have reached around 1220, then 1200, 1160 etc with F1 fault, so the kiln stops when it has been firing on full whack for more than 20 minutes without gaining 1 degree. When I have tested the elements by putting them on full heat, I can see there are cold areas on the lower element but no obvious reason for cause (I regularly gently hoover). I originally used the kiln for about 2 years without any of these problems. I have changed the elements more often than I should be. The last time I changed the bottom element and it is still not reaching 1240 so I'm trying to work out if there is another problem.
Bob, the cord is 1.25mm2 . I could drastically shorten the cable but I would be plugging it into a regular socket in our brick shed that runs from the house electrics and not directly on the main circuit so I was concerned this might be like running it off an extension lead (but perhaps not?).