Hi Cat
I'm afraid you do have to pay for an approved Sub Office relief, and they are like hens teeth, and they aint cheap, you can expect to pay £80 per day or higher.
POL do have a scheme that enables you to claim a substitution allowance, this is payable at a rate of around £38 a day, this helps but the main cost is down to you, if you contact Payroll they will tell you the exact figure and send you all the forms you need to apply to have your relief approved.
If you are a member of the Postmasters Federation, your local rep will probably have a list of names who are approved and trusted reliefs, I say trusted because not only do you have to pay for the relief you also have to take complete responsibility for whatever the relief gets up to!! So if you are not a Fed member I suggest you join. If you are a very small office it might be possible to close in order to take a Holiday, again a Fed rep will help you with this.
Hope this helps
Subbie