if you like the security of a constant price then go for it
you typicaly pay 10 to 20% on top for piece of mind lets assume 15%
lets assume your average bill is £1'200 plus 15% thats £1'380
now they know the prices for the next 3 months so lets assume in nov they anounce a 20% increase then in april a 40% increase now thats 60% and your probbably£400 better off!!!! well not realy
assuming you pay your bills monthly at the end off the month
july aug sep oct nov 5 months at £100=£500 furure payment 20% more =£120
so dec jan feb mar apr 5months at £120=£600
future payments are 40% higher than £120=£168
so thats may june at £168= £336
so thats £500 +£600+£336=£1436
by this example you would only be £56 worse off over a fixed rate even though the increase was 60%
now the actual rises are likly to be less and maybe earlier in the new year but who knows what is likly to happen
ps can someone check the maths as they where off the top off my head as my calculater is caput