I take it by horseboxes you mean stables rather than trailers/lorries to transport equines? If you have actual horseboxes as in vehicles, then rent that space for more of those and/or caravans, far less hassle and more money for the timescale you want.
If you mean stables, with 1.5 acres, does it have a water supply, good stockproof fencing, ragwort/weed free grass and of what quality/length is it? Is there secure access, what is near it for folk to ride or exercise their horses, any off road hacking within a few minutes will increase desirability for some, while others won't look without an arena. Is it an area you could park a trailer and find it there next day? Water I would say definitely required, power nice but not necessary to most potential tenants, I had various places without power but tracking water in every day wouldn't be worth taking on anything.
If it's really suitable for grazing then there is at least one person I can think of straight off that might want to talk to you, a Highland pony owner somewhere in Lancs but I'm not sure exactly where. What she'd pay for the whole I don't know but I would suggest that over renting individual stables to 4 people, which would involve you in a lot of hassle from their conflicting ideas, needs and methods.
PM me if you want. I'm nowhere nearby so no personal interest, just know of someone whose livery closed at short notice and where she went at the time wasn't ideal so I know she's looking.