I've just got a bunch of people to read and re-read the paragraph over and over again, and it says "government pay or 90% pay" followed by "if this is lower". It's taken ages to work out, but I think it's actually government set SMP unless 90% of my pay is lower than that. Right?
"For the first six weeks, SMP is paid at the higher rate, which is equivalent to 90% of your average weekly earnings calculated over the period between the last normal pay day before the Saturday at the end of the qualifying week and the last normal pay day at least eight weeks before that date. For the purpose of calculating average weekly earnings, shift allowances, overtime payments, bonuses and commission are all included.
The standard rate of SMP is paid for the remaining 33 weeks (or less if you return to work sooner). This is paid at a rate set by the Government for the relevant tax year, or 90% of your average weekly earnings calculated over the period between the last normal pay day before the Saturday at the end of the qualifying week and the last normal pay day at least eight weeks before that date if this is lower than the Government's set weekly rate."
Ah well, doesn't change my plans not to return to work, but it will mean less money.