Evening all,
I very much appreciate those taking the time and effort to help me navigate my way through this.
Apologies if I missed some information initially, but I was trying to be succinct, and it still ended up an essay!
To be a little clearer... the work I have in, and the pieces still to come, will be hung in a number of cottages currently being renovated as holiday homes. Renovation work on some hasn't begun yet, so will all be for next season.
This is why the owners are in no hurry for the framing, but realise as a "one man band" I will need to get started to have them completed "over the course of the year" as per their request.
They are still purchasing artworks for the walls, hence why there is no firm number in the pipeline.
I will be doing these pieces alongside my other customers orders, so it will, no doubt, take me a while.
I am working with the "middle man" who is dealing directly with the owners and interior designers (another reason the specs change and we have had some delays on decisions)
Perhaps it is evolving into a "not good business practice" on my part,

but I'm dealing with a situation, on a scale I've not encountered before, and is precisely why I have come here to ask for help from those with greater experience in this area.

Most importantly, I do want whatever solution I settle with to be clear to whoever may be keeping tabs on their finances.
I'm veering towards Tudor Roses suggestion of dividing the deposit by number of pieces and deduct from each invoice. This is what I did with the x10 pencil portraits cited in my first post, but didn't initially see it working in this instance.
It will mean I still have money coming in with each invoice, and I will repeat as/when they bring another batch of work.
Thanks again!