Just had a quick look in her book ... there's a range of dates when the price was 63.9p a litre, due to prices going up and down, etc.
The earliest date is 30/9/96 and the latest date is 29/11/98.
During these dates, the price fluctuated from a minimum of 59.9p to a maximum of 68.7p.
Incidentally, the price dropped to 61.9p until 19/3/99, then on 20/3/99 the price shot up to 65.9p and continued to rise steadily ......
Prices probably vary from region to region - we're in NW England - hope this helps :)
This price is for unleaded petrol by the way. I'm assuming you have a photo with the price of petrol in the background - it could be leaded petrol (I think it was still available around this time), which I think was slightly more expensive than unleaded.