Hello Marx
You need to answer this question from section44
To whom do the sub-contractors make their supplies/who is their customer?
If you are their customer (it sounds like this is the case) then there will be vat applied to the whole of your your sales invoice to the individual that engaged your firm.
The only way to avoid vat being charged is for these "subcontractors" to bill your end customer directly.In this case it is normally sensible for you not to get involved in the financial transaction at all, although you can have certain "disbursements" where you do get involved - these must meet the exact criteria per the earlier link.