Right, according to the gov't website itself, which was last updated on thursday, so reasonable to assume it's accurate.
'If you are on maternity leave, adoption leave, paternity leave or shared parental leave the normal rules for maternity and other forms of parental leave and pay apply.
Your employer can claim through the scheme for enhanced (earnings related) contractual pay for employees who qualify for either:
maternity pay
adoption pay
paternity pay
shared parental pay
If you are pregnant and about to start maternity leave you should start maternity leave as normal. If your earnings have reduced because you were put on furlough or off sick before your maternity leave started, this may affect your Statutory Maternity Pay. The same rules apply to adoption pay, paternity pay and shared parental pay.'
Basically, what this means is that employers can claim, through the furlough scheme, the cost of any enhanced maternity pay packages they contractually offer. So if your employer offers six months on full pay for example, they can claim that cost back from the government.
You will still get the normal maternity package you are contracted to get.
To get 80% pay instead of statutory mat pay, you will need to give notice and end your mat pay, which will mean, as I said, that you will need to be ready to go back to work at any time.