According to www.itjobswatch.co.uk a senior Java developer earns £70k. But can command day rate of £575. That works out at about double. Suggestion it is bimodal.
Though some industries (advertising publishing marketing) that formerly did not have to consider on technical excellence and where used to relaying of cheap Oxbridge liberal arts students with a 1st seem to want to pay temp technical roles at straight time instead of the real contractor day rate.