I have a similar scratch built model as the Talacre and I am using a Sealed Lead Acid battery.
This allows me to power the model without adding any extra ballast.
For new battery http://www.component-shop.co.uk/html/lead-acid.html
Click on the picture of the battery and you will open another window with battery dimensions, power etc.
The Component Shop give excellent service [often next day delivery] and cheap P&P.
(http://i124.photobucket.com/albums/p8/DickyD_photos/Coastal%20Tramp%20Steamer/DSCF2729-Copy.jpg)