We have a number of deliveries on our site. And more often than not the drivers have to work on top of the flatbed lorry, they will no safe system of work in place such guard rails, fall restraint systems, bean bags etc. In my workplace I would not have allowed this in my last workplace. However, my manager has stated that this is acceptable on our site, stating that if the contractor has regarded this to be acceptable in their risk assessment, then we accept their assessment. Quite often they will have basic working at height training. Surley this is a violation of the WAH regulations and WAH hierachy of controls and is not acceptable?

I sympathise with you on this one, however should there be a serious incident then you as the client could be held partly responsible if there are no control measures to reduce either the likelihood or severity of a fall - just saying. 

You could of course provide an air cushion to put around the delivery vehicles. It can be a clumsy and time consuming process, hence most don't bother with it - the choice is yours. Alternatively you could insist that all delivery vehicles must have fall restraint fitted...however you might end up short of deliveries.  

Been said before on the forum "Your Site, Your Rules" doesn't matter what the drivers company RA says they may be playing ostrich as the solution costs time/effort/cash - at the end of the day he who pays the piper picks the tune.

As these are flat bed what is being delivered?

Why is the driver on the lorry bed (i.e. why cant they work at floor level)?

Why isn't it ordered so that it can be off-loaded by FLT or other mechanical device?