closed circuit coil on 3 phase contactor needs to have time delayed off switch

The responsibility cant be put on their lap as you are the designer and installer of your work and where you cross the line in altering someone elses work then you take on the responsibilty that the previous is in compliance, what should have happened here was a risk assesment as well as an inspection of existing, that way you would have informed the customer that the existing dosn't comply and poses a potential explosion risk (if thats your findings), then you would have quoted for the necessary works to be carried out.
Having already started the works you have become responsible and the only way out if it is deemed to be an environment where explosive vapours and gases would exist is to remove any work you have done and cut your loses, if you know your customer well it may be just the case that you have a chat explaining your concerns and why you cant do the alterations they requested but either way it may make you look unproffesional, lacking competence etc in the fact you didnt bring the issue up at the beginning.

I would suggest you get a COSHH report on all products used within the paint booth, and if its the case they do pose risks of explosion then i wouldn't be touching any of the install as it exists.... the risks and liability is too high, also the very fact it seems you were originally unaware of these concerns would indicate you are not knowledgable regarding requirements and safety measures associated with these types of room which go beyong your green Reg's book.

Agree with the above posts about the use of unprotected electrical contacts in an explosive atmosphere, not something I would like to attempt even though i know somewhat about the use of tools in potentially flammable areas I do not know enough about electrical installations in this situation.

From an electrical wiring point of view I think the below work around would operate as you hoped. The left hand side is what you have at the moment the right hand side is replacing the 400V contactor with a 230V contactor, inserting a double pole momentary switch, hold on relay and a timer relay set for an (off) pulse a time after energising.
Just a thought and probably too much hassle to wire.
Can switch off if required with the off switch at any time but the on switch would not maintain on if pressed whilst the timer was operational.

Steve, are you intending to continue with this project? By now I hope you realise you’ve stepped in to a minefield.

Unless all equipment is outside the booth it must be Ex class. 230V control is not acceptable or practical for the control if you want a run on timer. As regards fire this is a two edged sword, depending where the fire is, running the fan could make matters worse.

You cannot use 400V control for more than a specified number of devices and comply with PUWER98.
Sorry but if you don't know the number of devices, and what is classed as a device, and what is allowed in the environment, then I can't help.
As has been said this is a minefield.

Tony,
Sorry to drag up old discussions, but "now" can you see what I was on about?
No offence.