When quoting for this type of work, remember to include holiday pay and sick pay if applicable.
Regarding invoicing, divide your price by 12, thus ensuring the same charge each month, this will cut your admin time on all your contracts, no need to change the invoice depending on how many full weeks are in a particular month etc. The idea is to work smarter not harder.
Regarding materials, your supplier should have two price lists available, one trade, the other retail, you should charge your clients the retail price, and take the difference that you actually pay as your profit from these items, this will allow your clients to see that you are not doing them, if they should start shopping around for consumable items. You should invoice these items seperately, at month end, so they are only paying for what they actually use.
You should also ensure that your T & C state that no credit is given for bank holidays etc, this will allow you to use these days as part of your staffs holiday allowance.
Hope this helps, regards,
Rob