I have washed a down s/bag & a duvet jacket by hand in the bath - it's hard work!
If you want details of this, post again and I'll let you know
However, since then I've had a couple of s/bags professionally cleaned and the results were much better - once by Franklin of Sheffield, which almost certainly was the firm used by Owen
However they are no longer trading and the business has been taken over by
www.elitecleaningandaftercare.co.uk of Rotherham
who also do an excellent job
The bag comes back like new; cleaned, and restored loft. It's bout £35, and thus hardly worth doing it yourself
If you're still not happy, then extra clothes would be the easiest & cheapest way to increase the warmth as you will probably have these already - socks or insulated bootees, long johns or microfleece pants, & some sort of fleece or insulated top
You can use a hydration bladder as a hot water bottle; the morning you will have lukewarm water and be well on your way to your first brew
I've just re-read your post. Do you really wash it 2 or 3 times a year!?. that's far too much, and will in itself damage the bag or quilt
I think I had my s/bag cleaned only after about 10 years
When you have washed a down item you must tumble dry it or the down will clag together. If you have simply hung it up, even if it looks reasonable you will not have restored the loft properly
From your posts I suspect that you might have only hung it up. If you have tumble dried it at home that will not be good enough as domestic dryers are not big enough to allow the down to be redistributed. At the least you should use a tumble dryer in a laundrette
I suggest you have it cleaned by elite and you should see a considerable improvement - they have a 10 foot diameter tumble dryer, and you simply cannot compete with that