2 x DeWalt DCB184 18V XR 5.0Ah Li-Ion Battery £99.98 delivered @ Powertoolmate

You can just buy one battery if you like but as the price is under £50 there will be a charge for shipping. You could add something cheap to the basket to take it over £50 to qualify for free shipping. They have Milwaukee inkzall permanent markers at £1.49 at the minute.

Features:

The new DeWalt 5.0Ah XR Li-Ion battery packs will deliver 66% more runtime than a standard 3.0Ah battery pack
LED State of Charge Indicator helps manage pack charging
Lightweight design provides the user with upgraded 5Ah Power without increasing the size or weight over the 18V 3Ah battery pack
No memory effect and virtually no self-discharge for maximum productivity & less downtime
Compatible with all DeWalt XR Li-Ion 18V Tools
Protection against overheat, overload and deep discharge conditions increases battery life
Excellent electrical characteristics
No memory effect
Specification:

Battery Capacity: 5.0 Ah
Voltage: 18V
Height: 65mm
Length: 110mm
Weight: 0.62kg
Compatible with: All 18V XR Li-Ion DeWalt tools