As long as it's sufficiently different to the original, you'll be ok. Leave out small technical details and use different colors, fonts, etc. It helps if you can simplify the map too.
Different authorities have different rules regarding the use of maps. Usually, you have to get a licence to reproduce a map "as is", and there are other usage restrictions imposed. I'm sure that, strictly speaking, just using an existing map as a model is bending the rules a little, but as long as it's not recognisable as the original, you'll have nothing to worry about. After all, maps can come from a variety of sources (OS maps, aerial photography, etc), so nobody can prove you reproduced theirs unless it looks remarkably similar!