I might be able to help here Daz.
I started with a twin barrel trolley (2 x 25L) from Peter Fogwill. I used it with 8 barrels in the far back and back seats of a 1991 Honda Accord Estate (somewhere between Escort and Mondeo size.)
Then I put a 175L tank plus 50m of microbore reel in the car plus two barrels and the trolley on the back seat. As you know with this trolley you lift out the frame and then slot one or two barrels, the battery and the pump in place.
Then I bought a backpack. And I modified the trolley (with a car jack!) to let me carry it in one of the barrel slots of the trolley!
So here we are like Fred Carno's Circus with three methods and pole all crammed into a medium/large estate! And it all worked.
I found that more and more I used the reel and hose and where I didn't I used the backpack and the trolley fell almost redundant. (And yet I had a job on a large factory where I did the fronts with the reel but the backs - about 100m away with no vehicle access I wheeled the trolley round. Also a school with a portable classroom across playing fields likewise.)
Then a French lorry driver wrote off the car (it was worth less than a grand) and I had a decision to make - repair it on the cheap or get another vehicle. I was doing well enough for Mrs. Gold to run an even older saloon car for the family and so I bit the bullet and put the insurance money down on a brand new van which I self fitted with what is basically my present system (a 400L plus varistream and 80m of microbore).
I never carried the trolley in the van (the frame and two barrels of which I gave away to someone on this forum - who never even put a thank you note through the door or a bottle of brandy or a cuban cigar or a voucher for a cruise round the med...

)
The backpack died and I never replaced it but last month paid £35 for it to be repaired (duff circuitboard bypassed) and a new switch fitted and now ...
I
do take the backpack (15L) and a 25L topping up barrel for jobs at the backs of terraces or up gardens where I used to run the hose through. This increases my overall capacity to 440 litres which gives me enough for a full day where I need it. It gives me the option.
The van ... (Doblo)
Hindsight tells me I could have got a better deal on the van by getting a loan and paying in full but for me the £500 deposit and the £170 a month were affordable and I have not regretted the decision and I have just decided at the end of the lease to pay the peppercorn rental of £170 a year and "keep" the van indefinitely as I've only done 24,000 miles in 4 years.