Steve thank you for reponse did indicate that existing head does not separate,also stated it was a GIOTTOS 3 way head
being carbon fibre.
Also a indicated social media site advised me purchasing a clamp which attached to the quick release plate inturn
allowed a acre-swiss fitting to complete the change over.
Not sure how else to explain the situation.
Apologies if you weren't aware but your head was available as a separate head and also as a kit with supplied set of legs (both via Amazon Uk) though it looks as though it is now a discontinued product. The legs I found it with are listed here:
Amazon.co.uk Hence, you almost certainly can remove the head from the centre column by unscrewing it anti-clockwise.
Note there is a model number for those legs, though I have no idea if that's the same legs you have, hence why I asked. It's common for manufacturers to supply various different heads (e.g. fluid, ball, 3 way etc) and legs combinations (carbon, aluminum, flip or twist locks etc) so that customers can mix and match according to their needs. They often also provide 'kits' that supply a head and legs set in the same box. I'm guessing that was the way yours was supplied?
Almost all tripod heads attach to the legs via a stainless steel threaded stud on the top of the centre column. Usually this is a 3/8" 16 stud on most better quality tripods. 3/8" is the diameter of the stud, and 16 refers to the number of threads per inch.
Some tripods (usually lower-end ones) will have a smaller 1/4" 20 stud attaching the head to the legs set (this is the thread commonly used for the camera screw or the threads in a spotting scope's foot). I don't have your particular head but do have a couple of Giottos ball heads and a fluid head, all of which use a 3/8" 16 thread size, so it seems fair to assume that yours probably does too.
I'm still not 100% clear why you would need to separate the head from the tripod legs if simply wanting to convert the top part to arca swiss. As you have already said this can be achieved by screwing an arca type clamp to the existing QR plate of your tripod. Clamps are cheaply available via Amazon and many other sellers, some for as little as £10 or so. This wouldn't entail removing the head at all.
However, if you wanted to replace the head entirely with another head, perhaps one with built in arca clamp, then you would need to unscrew the original head which might be difficult (or not?) They can seize onto the threaded stud if not removed for a number of years, but the usual way would be to first lock the panning knob on the head as tightly as possible, then unscrew the head counter-clockwise while holding the tripod legs firmly in place.
I don't know your particular tripod but some manufacturers fit several grub screws into the top plate of the tripod's centre column. These have to be tightened from underneath, usually with a screwdriver. They tighten against the base of the tripod head to stop it unscrewing itself in use, so if present you would need to unscrew these first before trying to unscrew the head from the column. Manfrotto tripods typically have three of these, but I'm not sure about Giottos? They might have even used thread-lock on the head which could make removal difficult unless heat is applied first before attempting to unscrew it.
Hopefully that is of some help, but if I've misunderstood then please ask again.
