It turns out that my house does not have a soakaway as it should. The rainwater goes into the foul drain, and as far as I can tell, has done so since the house was connected to the sewer in ~1970. I'm replacing my guttering and doing some repairs to my drainage, so I suppose now is the right time to rectify the situation. My main worry, apart from the cost, is that my sewage connection is so poor (very low fall, very shallow, needs rodding frequently in summer), that if I don't send my rainwater down it, I'll have constant problems.
I'm tempted to leave it as it is, but I'm also scared of getting caught.