There was a trailer the same as this on a farm were I was a student in 1987,didnt realise what it was back then but thought it was ex military,seeing your pictures brings it all back,it had a strange tank mounted on it which was tapered to floor,it was quite large & would prob hold water to fill the sprayer,i thought it must have been somesort of MOD water carrying trailer but having seen your pics I now know what it once was,the tank was removed & scrapped whilst I was working there,the trailer was kept,mite have a ride over & see if they've still got it.They also had,which I now know to be a brockhouse ex bowser chassis as a straw trailer which was called "bomber trailer",once I got the trailer bug a couple of years back I asked a retired employee of the farm if they still had it as I wanted it but he said I was a year too late as they burnt it (inc tyres) then scrapped it..wot a waste!!