Some interesting developments since I last looked on here. I got parasol to disclose how much of my employment costs consisted of NI contributions, but £24 a week was obtained very ambiguously. I believe they are calculating employers NI on the whole invoice, before expenses are deducted, and pocketing the difference between that amount and the correct NI figure.
I pushed them for some sort of recompense but the best I could haggle was 4 weeks margin free and 4 same day payments (although they will still be getting £24 a week from me!).
This whole debarcle has persuaded me to go limited in the new year (although that hasnt been straightforward). I considered employment tribunal, but decided against the hassle, although I'd be interested in how anyone gets on. I'd be willing to provide evidence of my particular case if that helps anyone.