The RSPCA is your first port of call though IMHO they're crap, as has been proven with other, well publicised, neglect cases. I'd also suggest that you call your local council and ask of they have a dedicated Dog Warden. Some no longer have one thanks to cutbacks. By law each council must have a "stray dog catcher" but for many these days that's all they have, which is someone who combines stray dog catching with other duties such as pest controller. In such cases animal welfare is no longer covered.
If your council has a DW they should be willing and able to go and visit/investigate, which will give your concerns more welly as they have links with the RSPCA and will be taken more seriously (yes, I know, it's a bloody appalling situation with our councils). My advice is that you fight and take it to the top - if there is a DW but he isn't pro-active take the issue to the Director of the council's Environmental Health department, if the RSPCA helpline is useless it won't be a first but come back here because I have the email of one of their lawyers who IME is good at passing concerns to the right people and getting something done.
Also complain to Environmental Health on hygeine reasons. You may be able to get something done there, in a roundabout way. If the owner is a council tenant complain to their housing authority. This is a legal "nuisance" and theoretically action can be taken on that score - I appreciate that this isn't your main concern and your worries are for the dog but if complaining about the hygeine risk does the trick it's worth approaching from that angle.
The problem is that although laws are being broken here they are those which are only usually taken up by the RSPCA as, although that organisation has no more power in law than you or I (despite their uniforms and titles of Inspector and Superintendent) they have the funds to take out a civil case against the owner where you and I don't. Unfortunately, as I said, they are notoriously slow and reluctant in some cases to act at all. I know of one well publicised case of 2 GSD left chained without shelter in the snowy Northern winter who they said were perfectly ok. The elder of these dogs died of illness a few weeks after being rescued not by the RSPCA but by struggling to make ends meet German Shepherd Rescue.
On that note, if all else fails, talk to them albeit that this is a Rott and not a GSD. The RSPCA hate GSD Rescue UK and the feeling's mutual but Jayne, the owner, will take up your case and fight with you, I'm sure. Jayne Shenstone is on 01568 797957 - give me a yell if that doesn't help as I have her personal email somewhere as well.
I hope it doesn't come to that though and that you get results from the RSPCA and your council. Poor dog. :(