You have a further complication here because you are dealing with Parcel Monkey, who are not actually a courier, just an agent. They are what's known in the industry as a web reseller, they don't own any vans or depots they just sell the services of a courier to the end user (you)
Remember the courier industry is largely unregulated in the UK, unlike the postal industry.
If you do not get any joy from ParcelMonkey from complaining/writing/etc then you will have to take them to court and prove that they or the courier they used to carry out the delivery) was negligent. Be warned, you shouldn't assume this will go in your favour.