Thank you all for your comments on this. I thought you might like some feed back on what I have done.
I have fitted a NP18-12B Yuasa 12v 17.2Ah Lead Acid Battery in the same position as the original battery, in front of the centreboard case. I made a box out of marine ply about half the height of the battery and screwed this on top of the wooden pieces in place for the small battery.
I also fitted a CTEK comfort connector. This does two things. It allows me to plug in my CTEK charger without taking the battery out of the boat. It also has three LEDS- green amber and red to show how much charge is still in the battery. I have fixed this with Velcro on the top of the forward end of centreboard case.
I can supply pictures of this if any one is interested.