I think you probably want one of the outdoor leisure or explorer 1:25,000 OS maps. Amazon do these. I think OL7 covers Windermere though you can check by Googling for Ordnance Survey and looking at their website. If you don't want to use Amazon, most good bookshops, like Waterstones stock them or most outdoor shops like Decathlon, Ellis Brigham, Cotswold etc. Especially if they are branches within the lakes. Within the national park you can pick them up in cafes, newsagents and petrol stations.