A little update, I sent my F2S to Sover Wong for checking and in his view, it had already been opened and repaired in the past and not up to standard. There was also so much fungus growing in the body that a replacement body would be a cheaper option to servicing it. This is a common issue with older cameras kept in humid climates (e.g. Japan).
So, I went with the replacement body option and now have a cleaned, calibrated and tested F2 body. The prism was also cleaned and recalibrated at the same time. All in all happy with the service and it was a fast turnaround for it.