Thanks to everyone for their lovely advice. I have an update.
We took her to the vets, who suspected an overactive thyroid and reduced kidney function. They want to do all manner of tests on her which would have cost £150+, which unfortunately we just didnt have so we took her home and were saving £20 a week from DH's wages to pay for it. She didnt get any better or any worse during this time.
Last night it all changed. She was having snuggles with us all evenings, and when we went to bed DH took her through to the kitchen (where she sleeps overnight) and put her in her litter tray. He said she did a huge wee, wobbled about, threw up a massive hairball and collapsed.
She tried to move/run away but her legs just wouldn't work. She started panting with her tongue hanging out, breathing rapidly then very shallow, and every now again would try to move but just couldn't. This all happened very quickly, within seconds she just went downhill. We called the emergency vet who thought she'd had a stroke
and recommened we make her comfortable - so we did. We sat with her for nearly 2 hours while she laid like a ragdoll, panting, mewing and crying every few minutes. We stroked and talked to her, kept her hydrated by syringing water into her mouth and made sure she was comfortable.
We left her for 20 minutes as our DS woke up....when we came back, she had gone.
It was one of the most heartbreaking things I have ever witnessed. She was lucky to have only suffered for a very short period of time, and we are glad she is finally at peace. But now my house feels very strange and empty...I keep thinking I'm hearing her meowing. I'm struggling with this more than I thought I would do and DH has been fantastic in supporting me, despite being devastated himself.
I know she was only a cat, and that she's at peace now. But she was OUR cat, our silly, soppy loving cat who would headbutt our hands and rub around us til we'd nearly fall over her. She was one of a kind and I miss her.
As I said above. Thank you for all your messages and advice a week or two ago.