I've changed the default preferences on my account in that it would refund on the 'annual renewal', which is in October - stupid - options were refund in full, refund over £75 credit (which was default) and offset future bills. I've reset to 'offset future bills', so the £500 credit wil help.
Further experiments - the hot tub
. Just replaced the impeller on the filter pump as the old one had just packed up after getting a little rattly (the spindle wears). On 'pump' only, it's consuming about 45w (the sticker on the pump says 50w so accurate). Pop on the heating and it's 2 KWh, so that's going to be a quid an hour, and will take 12 hours to get to temperature (from around 20c to 39c) - £12 at 50p a KWh. It was costing about £3 to heat for once a week use, which isn't too bad - heating is switched off after use, and takes 3 days to drop to 20c. Leaving it running at temperature will probably cost £6 per day
, averaging the heater being on 1/4 the time. As it stands I'll be leaving the pump on to keep the water filtered, then just switch off and drain come November when water temp drops below 10c. Bearing in mind we've had the tub for four years, it's going to be switched off this year. Just impossible to afford these days.