Has anyone helped you to try "laid-back" breastfeeding? It really saves your back. Instead of hunching forward over baby and cushion, and taking all the weight with your arms and shoulders, Mum can lean back comfortably supported by pillows or whatever (as if you were settling down to watch telly for an hour), and baby lays on Mum's chest (tummy down on mum), so that YOUR back is well supported and your BODY is supporting your baby.
Here's a link describing it:
http://www.llli.org/docs/00000000000000 ... eeding.pdf
If this doesn't make sense, then drop in to one of the local breastfeeding groups (Balham Breastfeeding NCT Cafe on Mondays, La Leche League Clapham on Tuesdays, or Gooseberry Bush NCT Cafe on Thursdays) to have one of the breastfeeding counsellors go through it with you, or hire an IBCLC for a private consultation.
http://www.nct.org.uk/branches/clapham/ ... al-support
http://www.lcgb.org
The health professionals at the NHS breastfeeding drop-ins should be familiar with this, but if they aren't then try the breastfeeding charities or an IBCLC.
Worth trying before you splash out for expensive furniture which might not resolve the aching back!