Get it checked Fred.
I persuaded my mate to take one of his TVRs to the rolling road.
It turns out it was 85bhp down on power, and drinking the juice.
Took a matter of minutes for the operator to find and remove the piece of air filter that had come off and almost completely blocked the inlet, plus a few other minor tweaks to restore to full health.
Not sure how new your vehicle is, but often these modern engines are very 'tight' when new, so may use more fuel for the first few 1,000 miles (one like that would also tend to run a bit hot though).