Do they have to have a reasonable estimate of when the work is going to happen, or can they issue the S20 and then sit on your money for years?
Their right to demand money in advance and the section 20 consultation process are two separate things. The freeholder's right to extract service charges in advance depends on the terms of the lease. Usually this will allow the freeholder to estimate the next year's service charge expenditure, and invoices in advance (usually once or twice during the year) based on their estimate. At the end of the year they work out how much they've actually spent, and either you have to make up the difference or they roll over any credit to the next year. That's not set in stone, but it's a common way for leases to operate.
If this is your case then you have to pay the service charges demanded in advance, provided the demand is valid and has been properly made.
Their right to demand money in advance and the section 20 consultation process are two separate things. The freeholder's right to extract service charges in advance depends on the terms of the lease. Usually this will allow the freeholder to estimate the next year's service charge expenditure, and invoices in advance (usually once or twice during the year) based on their estimate. At the end of the year they work out how much they've actually spent, and either you have to make up the difference or they roll over any credit to the next year. That's not set in stone, but it's a common way for leases to operate.
If this is your case then you have to pay the service charges demanded in advance, provided the demand is valid and has been properly made.
tl'dr - read your lease.