Check with these people.
www.lease-advice.org/
Your landlord has to give you a breakdown of costs and draw up service charge accounts for every year. That's a very big thing and if your landlord neglects it you may have trouble selling your flat.
From what I can remember they can choose their own contractor on anything up to £250 per leaseholder and there's nothing you can do.
Anything over that and they have to serve a Section 20 notice under the Landlord and Tenant Act 1985 with a consultation period. (Unless it's an emergency).
They'll give three quotes but they must allow you to inspect them and invite quotes and comments from you within the consultation period.
They don't have to go with your suggestions but if you do dispute it and it ends up in court they have to show they've paid proper account to your views and explain why they've rejected them. They're not automatically breaking the law by going with their own people but they do have an obligation to get the best value for money within reason.
If they break any of those rules they could be lumbered with all the costs over the first £250 unless they can show that the work was reasonable and their breach was just a technicality.
Also, if the work is poor or clearly overcharged you can apply to a court to waive all or some of the cost.
Anyway, Lease will know. It's worth asking them just for future reference. They are really helpful.
Lastly going to court (if that's what you ever have to do) sounds really expensive but it's not because it's usually hived off into a property tribunal where each side pays their own costs and can't claim from the other side no matter what happens.