Quite possible that the hornet was a wasp Queen.
Unfortunately and somewhat belatedly I have had to become pretty knowledgeable about wasps and would guess that what you saw was about 1.5-2in. long and as wide as one's finger.Of course it may have been a hornet but since we have had 5 of the buggers that I used to call hornets in the house in the the last few weeks it would appear to be the start of their season.
For several years our house has become the nesting place of choice for wasps in the locality. After a few years I got fed up with continually calling out the council control people, especially when they implemented a policy of charging, so set about dealing with them myself. My first plan was the macho action man decision to attack their nests in late summer when they had become particularly bothersome ,stinging people in the garden, and at times swarming.
Dressed appropriately in thick ski jacket ,two pairs of gloves on,a motorbike helmet and umpteen scarves, I crawled through the first trapdoor into the area above the soffits on one side of the house.The space I entered runs the width of the house,say 60 feet, and is roughly triangular with a maximum height and width of 3.5 feet and is criss-crossed by various joists and roof support beams.Armed with a range of implements such as long canes,water sprays,insect sprays,a torch , and several large empty plastic paint tubs ( to knock the nests into and then put a lid on), progress was slow on account of the equipment ,and my being 6 feet tall as well as being dressed like Michelin man.
It seemed that all the wasps had buggered off to one of their beer and wine orgies and I found several small nests the size of a tennis ball and dark grey in colour.I dealt with them and thought this is easy. I then entered the parallel area on the other side of the house and clambering halfway though a small gap between some beams lifted the torch and saw a beige-grey coloured nest that was about 2.5 ft long and almost the same in width. Nightmare come true and despite determining not to panic and to withdraw slowly I got stuckfor several minutes. End of the macho campaign!
I then searched the net for a couple of days and learnt that the grey nest were built by quueens who had died before they had completed building and that I had chosen the worst possible time of day and year to begin my assault. I also found out that the big nest was that of a previous year,they don't re-occupy. I got a guy in to deal with the active nests who told me that the unusually big nest could have housed tens of thousands of wasps which given that it filled a binliner seems scaringly possible.
Apparently the maturing fruit on the apple trees which we rarely gather because of wasps is what sends them crazy.Nowadays I kill any queens (wasps) I see and regularly check their nesting places at this time of year. As for bees they are welcome and strangely this year some have built a nest in a hole in the ground at the edge of the lawn which simply means less lawn to cut.