Can't explain it but you should have no problems unless you employ the same subbies all the time, almost as if they were your staff. If you employ the same subbies all the time, you can be seen to be their "employer" and will therefore have to give them employees rights, holiday pay, and pay emplopers National Insurance.
No you definately don't need to be a limited company. Though the contractors who still want to get out of paying their regular subbies, may say you hav to be a limited company again so they can get away with having not to employ you.
Are you a contractor or subcontractor???? If you are a subbie you will have no problems with the Inland Revenue. but if you employ subbies as if they were your staff then it may affect you.