dont take it so personally, it could happen to any of us. Perhaps try taking norman out for shorter walks closer to home, and then taking the mastiffs out for a really good blast to tire them out. Thus Norman will get peace and quiet both ways.
Just keep an eye on things, hopefully it was a one off, but you will find giving them a really good walk will help reduce tension.
Personally myself, i wouldnt risk putting them all in a confined space in the back of the car, just as if things did kick off in such a small space, injuries can occur, plus you wont be able to help as your the driver. Just give Norman special TLC treatment - he deserves it

Dont feel guilty.Ben gets 2 walks most days but there are times when he may only get 1 or even none if I am ill.As far as I am concerned your dogs have to fit in with you not you fit round them .Ben has a very good life so I dont feel guilty if he cant always have a walk plus anyway its good to vary your routine otherwise you end up being ruled by your dogs routine Things happen just dont beat yourself up about it

Thank you everyone for your answers, it's made me feel better and realise that things can't always run smoothly. My emotions are up and down at the moment which doesn't help.
But all the dogs I hope to think have a good life they have our undivided attention most of the time so like you've all said the odd missed walk or two isn't as castrostophic as I first thought!
Catherine x