Just got off the phone to South West Water (SWW), yes a sub meter (second meter) is the only option. as i said the amount used on the sub meter would be subtracted from the sewage from the first.
The meter is free and i can install it myself or get a plumber to do so or SWW will do it for ?100 - ?150. They're sending me some leaflets and someone will ring me from the company to explain in more detail in a week or so.
I queried it 1st October, they replied today. They have given me a helpline number which i'll ring in the next couple of days. It does say the second meter reading will be deducted from the first and not charged sewage. So i'll save some money.
My water authority (South West Water) emailed me with this:
If you feel that a large part of your water consumption is used to top up your pond, you may wish to consider having a second meter installed on the metered service, in order to register this use. This consumption can be deducted from the main metered use and charged for mains water only, while the remainder is charged for mains water and sewerage.
They say they'll give me a free survey for the second meter, but i guess i'll be paying for any meter being installed.
I'm using a Laguna Pressure-Flo 8000 UVC pond filter with an integrated UVC along with a Max-Flo pump and had no troubles at all apart from the in built UV didn't keep the water clear, so i added a Blagdon Pro UVC 32400 to the set up and got crystal clear water. You will have to change the UV every six months but i turn it off in the winter and just replace the bulb every april. (turn the UV off i mean).
i inherited a pond (5000 litre) which when i got going had very green water. i had a filter with a built in UV and it didn't do a thing. Bought a seperate UV and within a couple of weeks the water was crystal clear. As the other advice says your UV is only effective for 6 months abut i don't think built in UV's are up to the job.
i bought 6 shubunkins (about 2 inches long)and put them in a 5000 litre pond, after about a week 3 had gone missing, after another week one more had gone too. for the next 4 weeks all i could see were these last two. couldn't see bodies, didn't get ammonia readings, took water to lfs they tested it, found it to be fine, suggested heron took them. bought 3 more fish, put them in the pond, went back an hour later and counted 9 fish in the pond. have no idea where they hid but i couldn't find them. so don't know how big your pond is but they can be good at hiding.