Murcia-San Javier airport is located on the north shore of the Mar Menor. It used to have free parking with 900 spaces, but in the last couple of months, the barriers have been put into action with a parking charge of 8 Euros a day.
If you are going back to the U.K. for 10 days it becomes quite costly.After research, there does not appear to be any other medium or long term parking facilities in the area.
A taxi from Torrevieja is around 25 euros each way, and 30 euros from Cartagena.
Bus fare to the airport from Murcia is 7 Euros.
The airport has a good selection of budget airlines to and from a variety of U.K. airports, so now it is quite possible you could be paying more for your parking than your airfare.