The Lease is KING so read it.....if the lease does not require you to pay into a reserve and sinking fund then quote that clause back to the freeholder/mangeing agent. Also under L&T Acts in particular section 19 (1)any payment asked for in advance needs to be reasonable.
The lease will tell you when decorating works require doing normally every 3/ 5 years - have they done this? The reserve fund is not for emergencies it is for specific items of expenditure which will be in your lease..read the lease