Hi, I've got an iPhone 5 with Three, had it for nearly two years with no problems. However the last couple of days I've completely lost 3G, no internet connection at all. I've tried turning on and off Airplane mode, checking mobile data is turned on, turning phone on and off etc. WiFi is working fine.
Yesterday I was having trouble making calls and texts and my signal was up and down so I checked on Threes website and it said there was work in my area today (yesterday) so I thought it would be alright today. Checked again today and the works finished and it should be back to normal but it's not.

I do have an iOS update on my settings but it requires 4.4MB of data and I'm trying to avoid having to delete most of my songs/pictures just for this update so I've left it on there. Could that be the reason my 3G is off?

If you've got that little space a 4.4MB update can't be installed that may be the cause of your problems. (A photo is typically 1 - 2 MB so deleting two to four would clear enough space).

As for the works being finished today - they could have overrun and the site not updated yet (or a neighbouring mast is now being worked on meaning your local one is now overloaded).

Sorry I meant 4.4GB.. Just contacted Three and they confirmed the work has overrun so I guess I'll just have to wait. Thanks for your help

If you don't have enough space to download the update directly to the phone use iTunes on a PC/Mac to do it instead.

Assuming you have access to a computer running iTunes simply plug the phone in via USB fire up iTunes and once the phone has been detected you should get an option to update the phone. This should work just fine. Used it a couple of times to update my iPhone and know of a few colleagues at work who have done it as well.

And of course doesn't need a working 3G connection on the phone either and with a fast broadband connection would probably be quicker as well.

what iOS update requires 4.4GB?

iOS 8.1.2

What version is your phone currently on?

7.1.2 at the moment

The issue with your 3G is completely unrelated to you still being on iOS 7 so there is no need for you to upgrade if you don't want to.

Chenks must have a beautiful garden by now

Yes this is confirmed.
If you update OTA (Over The Air) then the iPhone needs enough free space to download, unpack and install the update as it has no way of instantly restoring all your stuff afterwards.
Everything would eventually restore of course but this would confuse the hell out of most users so Apple will only allow the update if the iPhone can do it without having to delete everything and then restore later.

If you do it via iTunes though, iTunes first backs up and syncs the iPhone to the PC/Mac.
It then factory resets the phone, installs the update and then restores everything again.
Very little free space is required.

I might be wrong, but I don't think that's what happens when you update via iTunes. As far as I remember, the only update that did a reset and restore was from iOS 5 to iOS 6.