Thanks folks
M & L - do you know we're I can get a copy/quote of the "law"
Google section 55 of the Town and Country Planning act 1990, this is the applicable legislation. Councils acknowledge its effect (which means a mobile shelter is not counted as a 'building' but then try to define what mobile means in sometimes extremely draconian ways. In reality if it is on skids, no more than 24x12 so it can be towed and has no floor and not used as permanent housing for anything they are on a sticky wicket.
It's essential to comply with the act that no flooring is put in, not concrete but also eg not imported aggregate, as this has been held by courts to breach top he temporary mobile nature (and anyway are works which require PP specifically) and also that the shelter is used as just that, without it being used to 'keep' the animals in eg horses in it as a stable, feeding supplementary feed in it etc.
NB if you use the shelter for horses (this may be more for other posters) and get into a battle regarding mobility of shelter council will often change tack to then pointing out that horses aren't agricultural and try and get you on the lands planning class as equestrian is a different class to agricultural. If so, bear in mind horse grazing IS classed as an agricultural use of the land even tho horses aren't agricultural animals but it needs to be just that, not them being stabled in the shelter or fed supplementary feed to any extend on the land, no show jumps etc.