Normally remove the internal arcitrave. Then using a hacksaw blade cut the nails / screws between the wooden trim and the wall frame often just 2 or 3 each side but check top and bottom . Remove sliding panel. Put some tape across the glass incase it breaks and then it should lever out to inside. Try not to twist the frame too much in the cavity as it may cause it to jam. Sometimes there is some flashing on the outside at the bottom that you can cut with a knife. If you need to tap out do not hit frame directly with a hammer as the glass will break use a block of wood between . Make sure you wear glasses and gauntlets at the least. JDE
It's fantastic to see you've already received some great advice from the knowledgeable @JDE. As he's mentioned, removing the timber architrave will be the first step. You'll find a pry bar useful in levering those boards off. Once they are removed, you might like to update us with an image of how the aluminium frame is connected to the house's timber frame.
I look forward to you successfully removing the frame, and please let us know if you run into any issues.
Thanks for your reply. I’ve successfully removed the window. It was a painful job - old-fashioned window really stuck in behind and in front of the frame in the cavity. No flashing ... I presume this one went in before the use of flashing? Just hoping I have measured up right for the replacement which looks like a much easier job to put in!
Thanks Mitchell. Done and dusted. Just waiting on the replacement window.