Turns out the dodgy running is the least of my problems. Went under it tonight and found that the sump is leaking! Only slightly, in fact I can't even see the split (it's round the back of a stud) but leaking it is. I do remember hitting a speed bump rather hard at the weekend.
Anybody know how big a job this is? Obviously I'll be ringing for quotes in the morning but is it a feasibly a DIY job?