Air blower keeps tripping RCD

[flyer]

I have a Victory Spa Hawaii corner bath with Air Massage System OOV-01. Installed in 2006. Mostly used by the grandchildren (who do play with the blower switch). I would say the bath has only been used approx 30 times in all this time.
The RCD started tripping out on the heater / blower, initially after approx half a minute running and this gradually went down to every 3 seconds.
I changed the RCD for a switched fused spur thinking there was a problem with the RCD.
I then ran the blower with no water in the bath for a full 15 minutes without it tripping.
Next day I filled the bath and when blower was activated automaticaly after oprox 10 mins it then tripped the RCD on the distibution board. I reset the RCD and it ran for 15 minutes without fault. I then emptied bath and when the blower cut in it tripped the RCD immediately. I reset the RCD and tried again, and again it tripped immediately.
Have you any ideas please.