We seem to have adopted a stray cat that we have christened Ermintrude.
We don't let her in the house because we already have two cats who are quite territorial, however we have been feeding her twice a day and have created a snug box under the deck for her.
Now we are moving and we can't take her with us.
Cat protection can't take her, they have no room. No-one we know wants her and we can't leave her for the winter.
The only other option is SPCA but we are worried because she's clearly an outdoor cat, albeit affectionate. How would she cope being locked up in a small space? Wouldn't that be crueler? She appear to have survived for a number of years without us (we have only been feeding her for four months and I estimate her age to be about 3).
Any advice appreciated.

Ahhh, that's a difficult situation there. Since you cannot take the cat with you, i suggest trying to find a good home for her. There are also many more places other than Cats Protection who are willing to take cats in, try searching for shleters in your area. It's winter now and the weather is bound to get worse and leaving the cat there will her no good at all. Can you not until you move, bring her in at night but confine her to one room, that way your other 2 cats will have rein of the house but at same time this cat is safe inside and very warm. If that's too much of a stretch...please make sure that this snug box is warm and safe and is shelter from the cold, strong winds and rain. Further hidden away the better it is. Try handing out leaflets to see if anyone wants to adopt her, place messages in newsagent or pet stores. And if you can place adverts in your local paper. Am sure a cat shelter will take her in since xmas is coming and the weather set to turn worse.