Sorry to hear of your woes Tinman. As everyone has said, it does sound like the battery. £100 fitted sounds about right, but check they're careful replacing it. The side boot lining it tight and easily creased, so ideally make sure this is removed along with any other liners required to get easy access. It is possible (just) to do it without removing the liners, but their knuckles wont thank them.
It would be a good idea to get them to check for a drain on the battery at the same time. If you have an old NavTrac fitted, once it's exhausted its own battery it will draw from the main battery irrespective of whether you're subscribed or not.
Starting the car is a huge draw on the battery, so I'd recommend at least 20 if not 30 miles to get a decent charge into it at least once a week. Battery conditioners are the ideal solution as others have said. Regarding jump leads, read your owners manual to get the correct procedure. Get it wrong and there is a risk you will fry your ECU.
Hope you get it sorted.
Regards, Miles.