I have £600, what should I upgrade on my setup?

Aloke Palsikar:
The 1st question I would have is what is your goal in short /mid term.  I assume you are interested to photograph some Deep Sky Objects like Nebulas, Galaxies etc.

Currently  you are using your Star Adventurer Mount along with your EOS 40D Camera and Lenses as described.  This is a good start

However I would recommend it would be worthwhile to invest your money in an entry level  Refractor Telescope and you can use it with your DSLR to capture images. There are several good products like Askar , Celestron etc.  Infact the Askar SQA55  also doubles up as a Telephoto lens.  Should fit in your budget I think. There are some good retrofitted, second hand buys available which work well.

As mentioned already investing in a dedicated controller like ZWO ASI Air could be good to manage your entire setup. Is definitely worth the price

Hope this helps

Clayton Ostler:

My Setup currently is:

Stock 40D

SW Star Adventurer

Canon 75-300mm Lens

SW Star Adventurer Tripod

So I am going to make some assumptions before giving my opinion. 

I am assuming you are managing your current setup with a computer or device of some type. 

If you are not yet doing that, I would suggest using some of that money to get something to do that. I love my asiair plus. 
Goto, PlateSolving, Electronic Assisted Polar Align, Imaging, Live stacking. Without these things I think the hobby would be too hard for me personally. 

I am not saying you have to get the ZWO device, you could use a laptop, or stellarmate, or build your own rasberry pi or whatever, but having a device/computer to run the show is pretty important in my eyes. 

Assuming you have that all solved, I would be looking for small used refractor. and small used guidescope/guidecam.

For me, buying an astro cam, but not having a scope to use it on seems out of order, I have used a camera lens on an astrocam before, but you are ultimately going to spend $150 on adapters and mount kits to connect to the camera and get it mounted on your mount, and I think that money would be just better spent towards a small refractor. 

For around $450 you could get a used at72ed and a flattener/reducer, and still have $150 left for a cheap small guidecam/guidescope combo. 

I sold my at72ed for $300 and there are 2 inch flattener/reducers for around $100 all the time. 
I see svbony 30mm with asi120mm combos on cloudy nights for around 150-175 all the time. 

Just my two bits.

Thank you for the reply, no I do not currently have a computer at this moment in time to control it but I was looking at facebook market place to buy one and I can get one for around 50 pounds.

SVbony sells a guidecam/guide scope bundle for around 110 pounds brand new, not sure what your thoughts on that bundle is but i think it's a pretty good bargain.

SVbony sells a 70mm Doublet Refractor Like the AT72ed for 230 pounds brand new, It's called the SV503 70ED If you were wondering. They also sell a flattener/reducer for 80 pounds too for the scope.

I Got a couple more questions.

Do you also think that the SV220 1.25 inch Duoband 7nm Filter would be a good fit for my setup? I also saw SVbony sells a version of the 585mc camera (uncooled) for 200 pounds too which I thought was interesting.