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Thanks for the thoughts. The tiles will be going anyway, so I'll investigate more thoroughly when I take them off. New units are going in too, they're 720+(137-175). Existing worksurface is at 905, so can easily sneak 10-20mm on that using the legs, depending on if the new surface is 20 or 30mm. I'm pretty certain I'm going to have to remove something more solid than plaster to make it work, but I still don't think there's a reason it's not possible.
Even if you can, you've got to make good with the tiles, the worktop will sit under them.
To clarify, I'm hoping to have the worksurface extend back over the window sill flush with the rest of the worksurface, covering the area where the tiles currently are. This is definitely going to require some surgery, my query is as much about whether this is a bad idea (don't think so?) as it is about whether it's possible (yes, with some difficulty/destruction).
hamrack-
I'm hoping to have the worksurface extend back over the window sill flush with the rest of the worksurface
In that case I'd say your best option is to see if you can do it with the legs . If you can't use ply under the legs to lift the units up the extra 20ish mm. Doing what you're planning I'd get a stone / resin type company out to template and custom make the worktops as it will be the most reliable way to get a good, long lasting finish
Bobbo
The limiting factor is the window frame.
There is some adjustment on the legs of kitchen units but it's closer to 20mm than 40. The legs are adjusted from underneath the units and are basically plastic screws, pull off the plinth and check how much thread is showing, more thread = less scope to increase the height. Remember you will need to check ALL the legs as you will need to lift all of the units.
Next is slightly more invasive don't do it until you know how much you need to take off the sill and if you're likely to be able to achieve it. You will need to take the tiles off. As the reveal is tiled it's most likely plaster under them if you're in luck it will be bonding and top coat. You may be able to take enough of this off to get where you want.
There is no guarantee that you can get this done. Even if you can, you've got to make good with the tiles, the worktop will sit under them.
P.S there is a rough and ready way to do it: shim up the feet of the kitchen units until the worktop is at your desired height, you will still need to do something about the tiles.