You should demand the refund from the letting company. There are essential terms in the contract - you pay for the trip, they provide the accommodation on a specific date. There is no cooling-off period for a holiday booking on a specific date, which then begs the question of whether this contract is void due to the current situation.
If one of these essential terms is not able to be carried out, in this case due to an external event being the pandemic, and making it illegal to travel there, you can demand a refund. You therefore need to say the contract has been frustrated and therefore void.
The key term here is "force majeure", or an "act of God" clause - meaning that if performance of the contract is impossible then the parties can agree to suspend their obligations - for the duration of the force majeure, or in this case, for as long as the service cannot be carried out due to the lock down. They would need a force majeure term in their booking conditions though and it should also refer to "pandemic", "outbreak", to be able to let them rely on this clause.
If you simply cannot go due to the rulings, the contract is frustrated – then you are due a full refund.
If you booked by credit card then you can make a claim - you can contact the card provider and ask for a refund under S.75 of the Consumer Credit Act 1974 - as they are jointly liable if you do not get the service you paid for. You can claim a full refund even if you only paid for part of the trip with the credit card as long as £100 or more was spent.
If you did not book with a credit card (or you did and the card company refuses to reimburse) then you should consider a Small Claim.
You should send the company a letter before action to demand repayment of the further payment within 14 days, failing which you will issue a small claim.
The attached letter is generic and needs to be tailored (and quote the above breach). The letter should be sent by post and email.
Assuming they do not respond or they do and refuse to pay, you need to register at www.moneyclaim.gov.uk so that you are ready to issue the claim in the event they dispute the claim and do not pay you. The website is very user-friendly and you would not need a lawyer to use the money claim site. Claims with a value of under £10,000 are classed as a "small claim", so legal costs are not recoverable and the matter may be dealt with on paper by a Judge, not at a hearing. A hearing may be necessary if the court thinks that oral evidence is required to dispose of the case.
You would claim the sum for the loss, the court issue fee (£105 based on a claim for £3,000) and court interest which is 8% calculated on a daily rate from the date of loss to date of court judgment. The site allows you to calculate the interest and add it to the claim.
If you are on a low income or have low savings (or in receipt of benefits), you can ask the court for a fee remission so you do not have to pay the court issue fee.
If you win then once you have the CCJ from the court the defendant has 14 days to pay in full. If they do not then it gets registered with the credit agencies after 30 days. You can also enforce the CCJ with the county court bailiffs or transfer the debt to the High Court for a small additional fee assuming the total amount owed is at least £600 and you can use the high court enforcement officers who have greater powers than county court bailiffs. The transfer fee is added on to the debt and payable by the defendant.
There are other enforcement methods which I can help with, including bank account freeze, charging order on their property (and then apply to force a sale), application to wind up the company if £750 or more is owed (if suing a limited company), apply to summons them to court for questioning, all of which can ensure you are actually repaid the money.
You may find they just pay you after receiving the letter before action – hopefully they will want to avoid litigation.
I hope this helps - please feel free to ask me anything else.
Have a good day,