Re-tiling behind already installed Aga

don't see why not probably just a very bit awkward

Yes it can be done as u said just a little more awkward. If u have access either side just fix a batten horizontally either side of the arga & put adhesive direct on tile & slide down to rest on batten. When adhesive dry remove batten & finish tiling. You probably wouldn't be able to completely tile behind arga unless there is a sufficient amount of space
Regards simon

Hi, This can be done, like you stated it's just awkward.

I would prefer to see the job before answering this in detail. However, you state you wish to change the tiles behind the Aga - if you literally mean that then the Aga must be removed to access tiles behind it. However if existing tiles are splashback only they can be removed quite easily provided you protect the Aga. The Aga is most likely already pushed back to the wall and likely has a small inbuilt splash back of its own and you could simply tile down to that - the downside here is that the tiles may break if later the Aga is removed for repair or replacement. Better to simply move the Aga out by 200 to 300mm (no need to fully remove), tile the splashback using a temporary strip of wood to tile up from (contact me for more details) then push the aga back. This is not difficult for electric Aga's but wood or oil burners would need the flue disconnected as well - I can provide more details if contacted.

I'd have to see it and I could say something more, if the problem is only limited access to this can be done you will only need more time to complete

It would depend on the tiles and how they are positioned but if they are easily accessible and above the Aga then there is no reason why they could not be replaced like any other tiles.

You are correct in saying its not much different, only its more awkward to tile behind the AGA. AGA to be covered over for protection and a bit of care and patience and the job can be done

if the tiling is level with the top of the aga and above ,no problem .aga,s are normally against a wall so it is not possible to tile behind them

I cannot see why they can not remove tiles above aga and retile

"I was told it can't be done", AKA, it's going to be awkward and I don't wish to do the job, or, they just don't wish to do the job. Most tradespeople will always want as much access as possible to every area.
Bottom line, ring around a few local tilers, and I'm sure someone will say ok. (I agree, moving the Aga is out of the question) But the next time a tiler comes to look at it, and says it can't be done, ask them why it can't be done.