I'm pretty sure it's either a mistake from the mobile company, or you haven't checked the prices.

Are you on PAYG, or contract?

If on contract, compare the cost of similair weekend calls, when you get the bill.  Also, was this call outside the free minutes

If on PAYG, then it could be mobile company making charges left right and centre, because they can get away with it.

One thing is for sure, if the number you dialled started with 01, or 02 and isn't Channel Islands, or Isle of Man, then it would be impossible to be charged 0870 rates for that call.

I checked your number, and it's in Lancashire, so the Offshore theory doesn't apply there.