Window sill replacement

You can buy pre cast concrete window sills & precast stone off the shelf from some builder merchants.

http://killeshalprecast.co.uk/building-components/precast-concrete-window-sills/

Usually it's not too difficult to replace one, does help if you have mate to help with lifting them into place, especially the longer ones.

You can even cast your own in place,make up a timber mould and cast a new one, fix either a piece of half round dowel or sash cord to create the drip in base of mould.
But, I would say it's easier going for ready made precast.

 

Whether they supply them in two pieces or one depends where you get them, I've had them 6' length in one piece, darn awkward manhandling them though even with help.

 

To keep costs down could you not cut something like these to size.



Would get the required slope for rain fall and a lot easier to handle.

 

Any builders merchant, various widths.

But, there is no rebar in them, probably too thin, 60mm down to 40mm, usually only available in short lengths 600mm & sloping over the whole width, so no rebate ( look at the profile of a window cill).

 

Granted not as good/strong as a purpose made sill, but at around £7.00ea for a 600mm length good value for money, I'm not sure rebar is totally necessary in a cill and if fact without one you wouldn't have the problem the OP has where the rebar has rusted and blown the concrete, the slope on the coping isn't that great so I feel that the small gap under the front edge of the window could just be mastic filled.

 

Have used once weathered copings myself for window cills, easy to grind a flat on back edge.