I always attempt to perform bike mechanics at home before I take my bike to my LBS. I recently decided to remove my rear derailleur, take it apart and clean it up, as part of a big cleanup of my bike.
I have put it all back together and back on the bike, but my bike refuses to shift onto the 7 largest cogs on the rear cassette (10spd ultegra with no problems prior to me taking it apart). Everything seems normal, but for some reason the shifter will just not move.
Any help will be greatly appreciated.

https://www.sheldonbrown.com/derailer-adjustment.html

Folks more capable than I will certainly chime in here, but I would go back to square one and try to reattach the RD and start over. One thing to look for the intial tension of the RD. It has an internal spring that gives it a certain amount of resistance when the shifter cable pulls it into position to shift. If that tension isn't great enough it can rub against the rear sprocket and get hung up. Happened to me. I found out that the vintage RD I had purchased had a very weak spring, so I bit the bullet and went with a new one.

If you have attached the shifter cable properly and cannot elicit any movement with the shifter itself, something is very wrong, indeed. Is the cable tight through the whole range of movement? Can you move the shifter itself? Don't know what else to say, but you might take some photos and post them here, that might yield some answers. Hope that helps a little.

Is the cable tensioned?

To do this more easily, physically put the chain and derailer in one of the middle gears, then lightly secure the cable.

At this point, try upshifting to the smallest cogs to see if perhaops the cable is now too tight, if so then turn in the adj barrel and/or loosen the cable attachment and let a little out until the thing will shift to the smallest gear. Normal adjustment can proceed from there.

I'm concerned about the derailer disassembly. Normally these aren't fully disassembled except after immersion as on a mtb. Hope you got everything back together properly, springs tensioned, pullies spinning, etc. and etc.

Firstly check that the derailer can be moved to the big-sprocket position, by hand.

The cable was becoming loose after a few changes, but is now tense. I have tried moving the derailleur across manually and it does move across with some force. Could it be anything to do with chain length? The shifter will only change to about 3 gears before it feels like it is top/bottom gear.

Did you change the chain? Here's what I would do: Disconnect the cable, remove the chain and make sure the RD moves correctly, and fully. Set you limit screws properly, then attach the cable and make sure you have full movement of the RD. After that is checked and confirmed, then put the chain back on and check for proper length and then you can adjust the shifting properly.

Ok, just re-tensioned the cable, and the derailleur is now changing into about 6 gears, I think it is now just a job of setting the limit screws correctly. Thanks for all the help. Will post if anything goes wrong.

You need to make sure the shifter is in the small cog position before attaching the cable. Work the smaller shift lever several times and pull all of the slack out of the cable. Add more tension as needed using the barrel adjuster.

Sjefferies: I'd suggest you follow this guide from start to finish, omitting no steps. https://www.parktool.com/blog/repair-...nts-derailleur If you work carefully and systematically you should be fine.

My guess is that the shifter is in about 7th gear while the derailleur is on the smallest cog. Then the shifter can only move 3 steps to 8th, 9th, 10th gear.

Disconnect the derailleur and let it naturally find its "resting" position on the smallest cog. You may need to move the chain down with your fingers to do this.
Then Pull the cable by hand and shift the shift lever so the cable comes as far out as it will go.
Hook the derailleur up again to the cable, with the cable snug, but not too tight.
It should shift through the whole range of gears now.
If not, did you play with the limiting screws on the back of the derailleur? Sheldon Brown will explain how to adjust these.