Thanks for your responses so far everyone,
Karminski, you mentioned about introducing hens of a similar age. Is this the only appropriate way to go when introducing hens? I say this because I was also considering getting some relatively young bantams (10 weeks ish), keeping them indoors until they're a little older and then introducing them to the older chicken, or would the age difference make this too difficult?
Babybantam and Chris, I love the thought of keeping the chicken in the house with us but I don't think it would suit her. She's always been a bit shy around people and I don't think the rest of the family would appreciate the mess! I agree with you about rehoming, she's been with us for so long we really wouldn't want to move her on now. She's also been bottom of the pecking order from the beginning as well.
So it seems 2 more chooks is the way to go (don't want to have more than 3 overall), it's just a question of whether to find ones of a similar age or get younger chickens. The family like the idea of going back to where we got the originals to pick out some young chooks, but introducing them to a much older hen may not be good, I've no idea.
Sorry to go on but could I confirm whether the older hen should be sleeping in the Eglu outside or in the house while she's by herself? Chris has recommended in the house for now but I'm not sure if she should be taken out of her usual routine?
Thanks again everyone.