Thank you for being straight forward , I have contacted a behaviourist about this and am just waiting to hear back from them.

I feel like I really let my dogs down and set sam up for failure with how I handled things today, and looking back if sam was a big dog I'd be seriously concerned for the safety of any tradies doing work on the house. At the end of a day he isn't a big dog (he is 3.5kgs) but this behaviour still isn't okay and I need to take responsibility for that and do the best I cant to resolve it.

I dont think Lewis played any part in instigating the fight as he just happened to be near Sam and ended up on the receiving end of what I'm thinking was likely redirected agression, as you said he was in the middle of a fight and just trying to defend himself.

I got overly confident with Sam's in regards to how he is with particular builder as whenever he comes over sam happilly greets him, trots next to him through the house, brings him toys and will play fetch if he gets thrown his ball. For the past couple months that we've been renovating he has ended up really liking the guy and is always excited when he comes by. I was suprised he got as worked up and aggressive as he did by the hammering today and I'll need to err on the side of caution from now on regardless who it is working on the house as well as adress the behaviour with the behaviourist. I'll also be keeping him away from the other two dogs from the get go whenever workmen come over.

Thank you for you help with this :)