Itchy dog - but no sign of skin irritation

My dog is a 10 year old spayed Lakeland terrier bitch, weighing about 8 kilos. She has had intermittent problems with itching all her life. The most recent episode involves biting and chewing her back and legs until there are small bald patches, but there is no redness, scaliness, scabbing or other apparent irritation of the skin. The vet wondered if it was musculoskeletal (pain in spine) and prescribed a course of oral steroids, but I don't feel they had much effect. Here's all the relevant info I can think of -

- She suffered from mysterious liver failure when about 2 years of age, had a series of long courses of antibiotics, eventually recovered although she has some shrinkage of the liver. I still try to limit the amount of protein in her diet (she eats a fish and wholegrain based commercial dog food which is the only one which seems to agree with her stomach). She has been pretty healthy since, and an ultrasound about 4 years ago revealed a normal-looking but slightly small liver.

- Within the last year or so she seems to have become quite wheezy, especially when relaxed/sleeping. She snores quite loudly at night. She also dreams and barks loudly in her sleep.

- She has cataracts in both eyes, somewhat more advanced than expected at her age.

- Sometimes I give her a 1/2 an antihistamine tablet when she is really itchy. It seems to help slightly, but I suspect it makes her sleep deeply and she doesn't scratch as much because she's so deeply asleep.

- I've tried various mild, natural shampoos and I've tried just rinsing her with no shampoo, but neither seems to have much affect on the itching.

- She has started to bite and lick her paws recently.

- She has her anal glands dealt with regularly, but sometimes they leak, and sometimes they fill very quickly after having been emptied.

- She has no sign of fleas, and I have used Advantage for the past year or so as it was thought she might have flea allergies.

- I can't see any pattern emerging to link the itching with heat, diet, unusual events, etc.

- She sometimes will refuse food in the mornings and will eat grass and have a very noisy stomach. This usually seems to happen if she hasn't had a bowel movement the evening before or if she has eaten unusually early in the evening.

- She is a very independent dog who used to enjoy time on her own, but she has recently become more affectionate and stays near me rather than going into another room on her own.

- She doesn't spend much time at home alone, but she seems to be okay when I leave her. For instance if I go out for the afternoons or evening it doesn't seem to make her anxious.

- She enjoy the occasional rawhide chew.

- She has a lot of stamina for long walks, but is equally okay on days when she gets less exercise.

- She doesn't seem to be in any pain, but over the years I have learned that she is very stoical and can be ill or hurt without showing much sign of distress.

- She is deeply predatory and obsesses over hedgehogs, cats, rats and mice.

- Walks, especially the last one at night, have become a bit of a chore as she is very distracted and will sniff the pavement and take ages to get walking steadily. We could easily spend 15 minutes just outside my door, as she refuses to move on until she has sniffed everything and she then insists on walking where she fancies instead of walking on down the street. I have several theories about this. She may just be scavenging for scraps on the pavement, it could be because the hedgehogs have begun to come out of hibernation and they are active at night (we've seen evidence of them and she is obsessed with them) or she could be hesitant because her eyesight has deteriorated and she is reluctant to walk in the dark. I get very impatient with her dawdling, changing direction and pulling, as she has always been pretty good on the lead.

I hope that is helpful and I would really appreciate it if someone could try to help. The vet just isn't much help and will undoubtedly just give her more steroids. I really feel for her when I see her biting herself like mad. She's been such a good little dog and I'd really like to do the best I can for her. Many thanks.