@old_scientist 

if you are going DIY then you may want to check the outcome of your efforts, I’d get yourself a handheld combination TOC/COD/conductivity/spectrophotometer. 
Id bet there are formal protocols for doing this, the Germans are very good at generating these. I do know they have a protocol for refilling and purifying the fluid that involves recirculating it through a mixed bed ion exchange resin until a threshold purity is met. 

just checked and it is VDI 2035

Whilst cleaning some very dusty and dirty solar panels this morning, I got to thinking…

Q. Why are solar panels covered with a grid pattern?

A. To provide the pigeons with a more challenging target area.

I am trying, so far unsuccessfully, to get an answer to the simple question namely is it legal (given the hosepipe ban) to use a hose to clean ones own solar panels. 

Affinity water have confirmed that it is legal for someone to do this as a business, but have not answered the question about DiY.  The actual legal notice would suggest that it is legal, unless solar panels are an artificial outdoor surface.  Obviously mathematically solar panels are a 'surface' but I would personally say that they are not a 'surface' in the way normally meant in the context of building.

The precise definition is

(k) Cleaning other artificial outdoor surfaces using a hosepipe

“Artificial outdoor surface” means any of the following:
a) any area outdoors which is paved or laid with hard or artificial material;
b) timber decking;
c) a quay;
d) a trailer designed, constructed or adapted to launch boats or other vessels or craft into water, other than a private motor-vehicle;
e) the roof of any domestic premises. “Quay” includes jetty, pontoon, wharf and slipway.

Interpretation: It is interpreted to include the cleaning by hosepipe of all exterior surfaces, by domestic customers. This includes driveways (both domestic and non -domestic); and marine infrastructure such as pontoons and slipways, whether fixed permanently in position or floating
.

After a short while that was followed by the 'power wars', where vacuum cleaners were sold on how much power they consumed (more being 'better') not how much suction they generated, leading no doubt to designs with deliberate inefficiencies. This bizarre situation was eventually ended by the EU that legislated for a maximum consumed power.  Of course some would describe what the EU did as unnecessary red tape!

They could also play Battleships with the pigeon on the neighbour's roof.

Whilst cleaning some very dusty and dirty solar panels this morning, I got to thinking…

Q. Why are solar panels covered with a grid pattern?

A. To provide the pigeons with a more challenging target area.

if you are going DIY then you may want to check the outcome of your efforts, I’d get yourself a handheld combination TOC/COD/conductivity/spectrophotometer. 
Id bet there are formal protocols for doing this, the Germans are very good at generating these. I do know they have a protocol for refilling and purifying the fluid that involves recirculating it through a mixed bed ion exchange resin until a threshold purity is met. 

Many many moons ago I had exposure to glycols in biochemical manufacturing. Its physiochemical properties are quite complex and related to its molecular weight and changes thereof. It will change from being a greasy like insoluble molecule to one that is fully soluble in water. It might be something best left to a professional as below certain concentrations it’s a rather nice substrate for bacterial and fungal metabolism and you probably don’t want those organic acid by products in your system if you haven’t adequately flushed it all out. 

Thank you. Yes, getting it all out of my system is going to be quite the headache given the volume (inc flushes) and safe disposal thereof.