David Slater giving you the benefit of the doubt about you hot set up, “taking your word” that it complies with all aspect of H&S including H&S for employees.
If you did have an incident with your set up, you will have broken regulations and most probably the law. (That also applies to of the shelf systems)
But in your case you will also be asked as to why you have this system instead of one made for window cleaners from a supplier. (Your answer better not be to save some money).
So what are the reasons for having this system over one from a reputable supplier? 
Can you prove your system is just as robust, suitable etc as the ones used by suppliers?
What tests were carried out on your system for the purpose of window cleaning?
These are just a few very basic and simple questions about your equipment. Fact is there are big differences between the types of system; using H&S for your own ends is definitely frowned upon, anyone with common sense will see through you comments instantly if there ever were reason to.
You can always tell them you know your onions! :
Oh my word Ewan....

I take it you've never come across the terms 'Practicable' and 'Reasonably Practicable'
You
REALLY dont understand H&S do you?...in all seriousness Ewan, get some H&S management training.
As some other posters already know, I'll be enrolling on the NEBOSH (Diploma) course this year and presuming I pass, I will have the equvilant of a BA (Bachelor of Arts) in H&S and as its an Honours Degree course....that will get me membership of IOSH at Graduate level.
H&S is my thing...I love it!! Its interesting, stimulating and challenging.
If you want to go 'head to head' with me on H&S you are more than welcome. As its such a broad spectrum, I think we should narrow it down to specifics.
What would you like to know specifically?
Would you like to know how I'm controlling the hazards?
Would you like to know what measures I've put in place?
Would you like to know how my system has ergonomics built into its design?
Would you like to know my reasonig behind gas as apposed to Diesel from a Health and Welfare at Work standpoint?
"buying a system" is somehow better than designing the sytem yourself??

One of the foundations of good H&S is discussion with staff/owners on the best design/user operation of equipment. You seem to have this strange idea that buying a system has somehow absolved you from any responsibility.
Let me set you straight there Ewan - I have major "issues" with most manufacturers of wfp systems - they require the user to climb (or lean) in to the vehicle to reach controls. You do realise if you fell from the vehicle while changing resin or turning the system on (or off) this would breach WAHR....which we seem to quote so often

Do some research Ewan or get some decent qualifications. Then come back to me and tell me your "bought system" is "safe"
Anglocom,Not sure I'm allowed to mention other sites. Some members of CIU have been banned for less!!

NATHANEAL (!!!)

has seen my pics. So has chrisyg..... and quite a few others who belong to another guild of window cleaners