welcome Toby!
You probably already know, but your freeholder cannot make you pay more than ?250 in any one year for building works, unless they have properly consulted with all leaseholders - in a specific way set down by law (S20 of Commonhold & Leasehold reform Act 2002).
And sinking funds for future works are fine, but you should be given details of what the sinking fund will be used for (ie a 10 year works forecast) - and funds held in sinking funds must be held in a special, interest bearing, bank account.
If you ever have any queries or problems to do with being a leaseholder, service charges etc, I recommend the Leasehold Advisory Service - a government funded service providing free and impartial advice. http://www.lease-advice.org