MR2 mk2 speedo 'running fast' ?

Hello all, hopefully someone can help/confirm the fault and fix.

I have a Mk2 Revision2 which was purchased in Oct last year and is used on weekends so I have not spent a lot of time with it yet. However, I believe the speedometer is inaccurate and appears to be running 'fast' ( better than slow I guess ). I have been driving for 38 years and have a pretty good idea of approximate speed thus when travelling at what I believe is around 30mph the speedo reads between 40-45mph.  I have been aware of this for some time but equally noted that the speedo quickly returned to zero as I stopped, however for a while yesterday whilst stopped at traffic lights the speedo hovered between 15 and 20mph. Approximately ten minutes later it returned to zero when stopped and continued to do so for the rest of the day although while travelling it continued to give readings that appeared faster than I was actually travelling at.

I have noted on an MR2 specialist site. ( MR2-BEN ) that the Rev2 onwards has an electronic speedo and I guess there lies the problem. Has anyone experienced a similar problem and if so can you confirm the repair. At the moment I am assuming I need to replace the electronic gearbox sensor which is about £300.

I will be most grateful of any and all helpful replies and thank you in advance.

Kind regards, Simon.