I see you are in the UK; is this a UK model, a USA model, or ??? On the back of the mower deck should be a serial number, usually in the form of "MXXX-1234567" that will help me nail down which exact mower you have; here's a photo of a USA-spec HR194:
Here's a sample shop manual page for a USA spec HR194's Roto-Stop; this model had an "A" version of Roto-Stop and a "B" version. "A" versions were simpler, and "B" versions have more springs...