We were in the same position. We aimed to match the WMC with local housing benefit rates which you can find online.
The Clerk had individual meetings with each resident to assess their position, most being on housing benefit already and explaining to them that the council would cover the increase, which it did without any issues.
Provided each resident with the appropriate formal letter and gave them about 3 months date wise as the council can sometimes be quite slow in responding, also to avoid unnecessary stress on the residents.
For those who are proud and do not want to claim housing allowance, it is a question of guiding them and making them understand that, for the financial survival of the charity, this has to be.
Good luck!