Hey @adj, welcome to the thousands of people in this city querying and often successfully disputing service charges
I would recommend as others have suggested getting in touch and joining the wider group as it looks like this is something that has already been organised.
It is always worth knocking on neighbours doors, even if you don’t already know them, to see if they are interested in joining the action. Most of the time everyone is in the same boat and there’s no harm in asking.
You are entitled to ask for a breakdown of costs and expenditure from Lewisham Homes so I would request that first.
Not a lawyer, but do avoid withholding all your service charge; it might land you in more trouble than it’s worth, some leases are particularly nasty in this regard. But if it comes to it, you can have a look at how well Lewisham Homes does at the First-Tier Tribunal where most disputes end up if they cannot be settled “amicably”:
Myself and some other leaseholders took our freeholders to tribunal earlier in the year after failing to get any sense out of them, and we received a pretty decent outcome.