What render to use around upvc window frames fitted flush to timber frame.

Hi all, we have a little problem with a little water getting on the door trays mainly

We have upvc windows fitted flush to the outside of a 9x2 timber frame. 

we have rendered reveals from the back of the stone work to the upvc windows and doors. (Around 3/4” worth of render pointed over the window / door frames 25mm ish.
 

we have taped dpc around the windows and

doors using butyl tape, tyvec tape below that. 
 

we have a small bit of water ingress in what we think the point where the render meets the cill. We think as we struggled to achive a good finish with dpc & tapes etc around this section - we are getting some water getting onto the tray underneath the window and door frames.

unfortunately i didnt go to the expense of using lead trays so we have thick dpc home made trays instead, meaning i think we should do some remedial work. 
 

so my question is- is it worth hacking off the render and replace with a waterproof pre mixed render system? 
 

Keep exiting render, and add another 12mm of a water proof render over the top again?
 

And or Use a permaseal type of product (masonry cream) onto the existing render. - then potentially add a thin layer of pre mixed waterproof render over the top?

Thank you in advance, would like to hear peoples opinions. 

Thanks