So, it seems that Payroll Liabilities simply don't work the same, which is inexplicable. There is no 'Pay Liabilities' option, so give up looking for it, as it simply isn't there.
I got talked through it (the guy learnt a lot from me too, as I had payroll liabilities to carry over, as I had a member of staff who received SSP just before the change, and another who had a tax rebate, so both amounts had to be carried over and he clearly had never done this before, so I ended up doing the working out!).
Here is what we did:
It was fine once I knew what to do, though what a faff by comparison to how easy it is in Desktop!
For anyone who has liabilities to carry over, as I did, we solved this by making a journal entry which transferred the amount from the Payroll Liabilities of the imported Desktop version to the current version - I forget which way the transfer was now, though I just played with it until it worked and matched what was on the Desktop version before the transfer. Whether this was the right thing to do or not remains to be seen, though it appears to have done the right things at my end, at least.