Thanks Everyone..
I reckon I was doing about 110-120 mph
The motor was well warmed up
What's a map-whack ???

 

You could try putting the car in sixth gear while sitting stationary with the clutch in and rev the car past 6000rpm just to see if it is an ECU thing with sixth gear.

 

Are there speed governors in your country for passenger vehicles?

 

There isn't a governor in the S here in th US that I'm aware of. Not sure about where the OP lives however. That may be your culprit, or the map sensor is a little loose. You're sure the oil levels are good? Since you have an AP1 oil levels are EXTREMELY important. Check often and check both sides of the dip stick. If it says you are full on one side and low on the other go by the lower side. Air filter wouldn't cause the sudden loss in power, still a good idea to change it though.

 

I live in Ireland but the car was originally a UK car
I'm not sure if it has a speed governor... How would I find out
I will check the map sensor to see if it's loose
Waiting for the service items to arrive, my local auto factors had to order the air filter and spark plugs
Not many S2000 in this country !!
Thanks for all the advice