"...what's the legal position on billing the inland revenue for my time completing their form when thier guidelines suggest I don't need to have a form anyway?...."
Depends how you cost your time. As an independent consultant, I would just enter a business expense calculated at €48 per hour - my normal consultancy fee.
Just wait until you're no longer on PAYE! The buggers actually bill you in advance for the next tax year on the assumption that your income will be the same as for the previous one.
A real incentive from Fatty Brown to encourage people to be honest....