If the contractor is employing people to work with this equipment, I can't see how they could *not* have the legal duty for the maintenance of it (even if this maintenance is actually done or arranged by someone else such as the client who owns the equipment).  That's because they are the employer of the equipment users and it is the employer that the duty falls on.

But the contracting company had no say in its specification before purchase ergo even if they know the task there was no opportunity for them to design out potential hazards by selecting possibly more appropriate engineering controls which may have been cost options the client has refused to pay for.

The client should not have made the purchase if it was not for their direct use.

Would you purchase ladders for use by the window cleaner or make it part of the contract they provide suitable equipment?

PUWER places duties on the User of the equipment, not the supplier or owner.

If an 'employer' agrees to use a peice of equipment they are the 'User' so take on all the legal duties under PUWER like ensuring it is suitable for the task, CE marked, suitable training is provided etc. 

Clearly they also take on the reasonibility (and legal accountability) for ensuring the equipment is suitably maintained and fit for continued use. This cannot be transfered through contract. You can debate who should organise and even pay for that maintenance, but the User (employer) of the equipment must ensure it is completed as required if it is available for use by their employees. 

But isn't there more than one employer involved here?  The employer referred to in the OP is the client (Company A).  The client makes equipment available to another employer, their contractor (Company B).  The contractor expects their own employees to use this equipment.  Then it is Company B that is the employer with PUWER responsibilities.  Company A does not have an employment relatioship with the employees of Company B.

But Company A specifed & purchased the equipment (one can argue without the involvement of the actual hands on employer Company B). If I were Company B the equipment would be rejected as it is my employees I am concerned with and I have legal responsibilities towards.

I do think the point is either being missed or perhaps not explained clearly.

The item is a Sand Blaster this is not portable equipment so would need to be installed as a fixed installation on the premises and would therefor require its PUWER Assessment to be completed by the persons asking for it to be installed.

Company A (The Client/Employer) provided the equipment, so I am assuming that they have installed it on their premises and are allowing it to be used by company B (Subcontractor/Employee) and that it wasn’t delivered on a pallet for company B to install themselves.

If company A installed it for the use of company B, then Company A are required to have it PUWER assessed before handing it over. If not when company B conducts their risk assessment prior to using the equipment they should identify it hasn’t been PUWER Assessed and either get company A to complete this on their behalf or have it done themselves.

Either way PUWER should be captured.

The question originally asked who should carry out the ongoing maintenance and inspections for ongoing compliance with a PUWER assessment that has been completed, not who should do the PUWER assessment in the first instance.

In this instance it is reasonable to expect that company B take on this action and responsibility as they have its exclusive use so are the only persons using the equipment, therefore they are obliged to maintain and inspect it.

Company A can rightly pass on this responsibility as long as this has been explained to company B and understood by both parties.

Company A still have liabilities though as the equipment is connected to their energy supplies and used on their site by an employee e.g. company B.

If an accident were to happen they could be found to be partially responsible down to lack of management and supervision of their contractors under the health and safety at work act.