Personally, I would say a ‘deposit’ of £2900 is somewhat excessive. But it does rather depend on how much of that is for your chosen optional extras/package (which the dealer can ask for).
Motability do state:
‘Some dealers do ask for a holding deposit when you place your order, but any deposit paid will be deducted from the Advance Payment you pay when collecting your car’.
https://www.motability.co.uk/about/how-the-scheme-works/payment-and-cost-for-cars/
Albeit it is a quite ‘grey’ area – some dealers do and others don’t
A worry is that, as the deposit doesn’t form part of Motability’s Advance Payment, it is held by the dealer (until it is deducted from the AP on collection date) if the dealer ceases trading or goes into administration/liquidation during the intervening period – particularly if it is over a period of months, you may lose out. You would be just another unsecured creditor.
It is one area Motability kindly ‘absolve’ themselves from as they do not guarantee to indemnify you should the dealership cease trading, hence I personally would refuse to pay any deposit and go elsewhere.
The only other recourse is to totally demand that any such ‘deposit’ is paid by credit card, thus you may be covered by Section 75 or 75A of the Consumer Credit Act. Even paying by debit card can be risky as a ‘chargeback’ is subject to a 75 to 120 day ‘window’ once the transaction is carried out. Thus, if waiting months, you could exceed these limits and be left not being able to ‘chargeback’ if the dealership ceases trading.
Sorry to sound gloomy, but in these uncertain times, I do think a £2900 deposit is somewhat unreasonable for the dealer to ask for.