PRP is not just about pay, its also about satisfaction, keeping your best staff, it is also linked into personal development, that is, learning or enhancing skills and knowledge.
When this works, both employer and employer benefit, as it addresses personal values.
When it does not work, the best staff leave, the very best will already be making plans even now, the remainder become dissatisfied, they have no peer leadership.(sound like a contradiction in terms, but actually peer behaviour is very influential)
This company is on the slide, I would be interested in knowing how well it is meeting the Service Delivery Agreement, being a PPP company there will certainly be one.
My first inclination is to think they are having trouble meeting their obligations within budget, at I would think this is leading to your situation.
The figures should be readily available, try find anything on recruitment and retention, it will help your case if you collect evidence.
Stuff you are interested in are things such as rates of changes of over-application for available posts, the amount of sick and the trends, the duration of sickness, classicly, increases in short term sickness increases and indicate a falling commitment by the workforce.
Staff turnover might be useful, but you have to be careful to allow for rationalisations.
Numbers of disciplinary hearings etc also give a useful indication, especially when they are based around race relations, or unequal treatment of disabled, gender, or other equal opportunities matters.
This sort of thing is irksome, because PPP companies should be net wealth creators, they should be more flexible than the public sector, able to offer greater rewards to good staff, more efficient and as a result provide better service at lower cost, but such things can only be done with willing staff.