Not sure about getting maps, but your drains should run in straight lines. From house to main drain/manhole.
If you follow your soil stack from the toilet, to outside then walk in a straight line from this point directly to the manhole/main drain. This will more than likely be the route of the pipe.
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Also if you dig near the house the drains will probably be around 1.5 - 2ft down, the further you get towards the main drain the deeper it'll be. If you lift the manhole cover and look where the pipes enter the chamber it will probably be around 4ft down.
You can get plans of public sewers from your waste water company, ie Thames Water, Southern Water etc. They charge about Â£35 I believe but only show public sewers. Your local building inspector may have an idea if any houses near you ahve had works carried out.