IR35 is explicitly contract by contract based... In theory (and in practice) you could be working at 3 places at the same time and 2 of them would be outside IR35 and 1 would be inside it...
There are a lot of old wives tales and myths around IR35 http://www.contractoruk.com/ir35/top_ten_ir35_myths_debunked... shows a few of them. And sorry for posting links to the same two sites but that is where the real information is from people who have operated this way for years...
You're right and I glossed over most of it but the killers are concurrency, substitution and fixed deliverables. With concurrency, if you can prove that during the contracted period you had billable work outside the contract then that goes in your favour if you get nabbed and the contract is borderline. With substitution, the contract should state that you can provide a substitute on demand so you're contracting out services and yourself. The third is fixed deliverables; that's always a killer. Rolling contracts as non distinct labour are how to get shot.
For ref I had a small battle on this front and concurrency won it for me.
TBH get a good solicitor and get them to take a look at the contract paperwork. That's better advice :)
Concurrency was part of the HMRC's Business Entity Tests which were so hilariously inaccurate compared to IR35 actual rules its was funny.
I guess if you had a small battle and that won it that battle wasn't with HMRC but instead with the Cabinet Office or another government department / quango...
IR35 is explicitly contract by contract based... In theory (and in practice) you could be working at 3 places at the same time and 2 of them would be outside IR35 and 1 would be inside it...
There are a lot of old wives tales and myths around IR35 http://www.contractoruk.com/ir35/top_ten_ir35_myths_debunked... shows a few of them. And sorry for posting links to the same two sites but that is where the real information is from people who have operated this way for years...