It is a common misconception that Service History only counts if there are stamps in the book. The thing that vexes me when I go to buy a car (believe me I do it at least 3 times a month) is when I ask for history, I am told:
"oh well I have carried it out myself........every 3000 miles"
or
"My brother in laws a dab hand, he always does it for me"
The I say, "yeh, well the parts didn't fall out the tree's did they??? where are the receipts???"
You do not need stamps in the book all he time.
A receipt with the vehicle registration it is fine.
Sure, it is not proof that the parts went into the car, but if the car looks well looked after then you will be taken seriously.
El