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First of all, thank you so much for the prompt reply!
I had no idea that there was a centrifugal switch within the motor itself so I will have another go at that as you suggested.
I should have also mentioned that I added an in-line booster fan to help circulate the air to the upper level of the house. I took the power for the additional fan straight off the control PCB using "piggy back" spade connectors where the main blower motor gets it's feed. The start problem seems to be less prevelent if I switch off the booster fan (I added a rheostat to control the booster fan speed). I wondered if this was part of the problem by wearing out the relay contacts prematurely due to the additional load?
Regarding the plenum stat. There is no adjustment that I can see on the unit so do you suggest replacing it?
Also, when I looked at it again today, I found that there are actually what appear to be another stat / sensor in series with it, with the 2nd one being a TO3 metal can configuration mounted on a part of the furnace where air is drawn in and mixed with the combusted gas. Both the plenum T/Stat and this second sensor appear to be "normally closed" when measured with an Ohm meter with the furnace off.
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