Reentrant wrote:

I've accepted redundancy but my about-to-be-former employer wants me
to work part-time under contract on a self-employed, sole-trader
basis (ie not as a ltd company)

1) They've offered me a rate of about double what my salary was - is
that about right to compensate for no holiday, sick pay, pension, NIC
etc?
2) They don't mind if I work one day a week or one week a month. Is
one or the other better for NIC purposes?

Not sure HMRC would let me register as self-employed for one week a
month and deregister for 3 weeks to avoid 3 x Class 2 payments. I
already have enough NIC years for a full pension.

If you want to keep it simple, you might be better off just to use an
umbrella company.

For a small commision they will supply a pay-as-you-go tax efficient
accountancy service.

They will invoice your employer for your work done, calculate and process
whichever deductions are due and then pay you the net value of your
earnings.