On the basis of the advice here--thanks to all--I am splitting my dose of gabapentin throughout the day, three times per day. Thanks Glenn and Nilram.

I've noticed that I am often worst while teaching late in the evening--at 7-9pm--and this might because I took my dose in the morning and by then it has worn off.

Useful info about extended release variants Nilram--new to me.

Cheers

Paul