It all depends on whether you need a battery that is capable of starting the outboard engine. The Maplins batteries are not likely be designed to provide a high current.
Electronics supply stores (Maplins) will carry a lot of different sized SLA batteries the smallest being about 7AH which would do the job but bigger might be better. On the other hand a small car battery might be cheaper if it is on sale due to huge quantity sold. Especially as this will have a much greater capacity eveen though it is designed for high CCA.
good luck olewill