yeah there are a few, as mentioned OS map will display official OS maps but is dependant on an internet signal as the maps can not be cached locally on the phone (although there is a 3g signal throughout most of the lakes).
the second choice is oruxmaps which is free from the market place same as OS map, again orux maps can be used online with google maps or there is an option to use locally cached files but be you need to make them your self and load them on the phone (you need to be quite tech savvy to accomplish this). The good thing about orux maps is the huge amount of features included, you can load routes and tracks onto the map (using gpx files), track your route as you go and view numerous statistics of your walk, i highly recommend it.
The third and probably the best application is the newly released viewranger for android (although not free you can download a 15 day trial from there website), this is a fully featured gps map system, meaning you can purchase the maps and load them on the phone without the need for an internet connection (lakes are about £60 in 25,000 scale), obviously the fact the maps are stored locally is the winner with this app, its fast reliable and has the best mapping software you can get for walking, That said its only just been released as a beta test so a lot of the functions are still missing such as gpx file loading and route tracking, but they will come in the final release of the program.
i just completed a 33 mile hike in the lakes using oruxmaps and was pleasantly surprised with it, although next time i go i will be using viewranger as having the ability to view official OS maps without the need for an internet connection is the winner for me.
Ow and if your worried about battery usage you can get extended batteries from ebay for £7 ish each, i have 4 2600mah batteries from china and they last a long time, with the gps turned on and the screen being used for most of the day i still had around 77% battery life at the end of the days walking.
