I had a similar situation and in my case the Structural engineer returned as a matter of professional pride. His attitude was that he made good faith recommendations about what was observable at the time and later when he was able to inspect further he was happy to refine his advice further.

As a structural designer it is a responsible profession and I am surprised that he did not offer more assistance in this case. There could be any number of reasons for this. Did your original quote include detail design? I would look for another SE. Builders tend to think they always know more than SE's but its the SE's reputation and expensive insurance that is on the line if it is not done correctly. A collapsing structural support can be a very serious matter. This time find out what the calculation cost includes. If it is a calculation then that is all you are paying for, not investigation time and supervision or inspection would not be included.

We did get a connection design from him however I’m not sure how helpful it was but i think one contractor is used to working with a SE he knows and lays it all out for him including being on site and telling him how to do it. I think both him and the other contractor expected the calc document to say which wall the steel is to go from and to and which way but it doesn’t. i suppose essentially we got what we paid for but it has been so difficult getting a direct helpful answer which always leaves us with more questions than answers.

whose responsiblity would it be to knock holes in walls to see extent of existing steels? and if it is based on observable at the time, do we then have to go back to him to tell him that the existing steel doesn't span the entire width to get his opinion on a further support as he has only said it is an assumption.

we paid for a site visit (and he told us for measurements but at no point did he take any measurements nor note anything down). he also quoted his design fee and steel connection fee. he didn't offer anything else. we are new to this so weren't aware of anything else we needed. at no point did he offer to come back out.

is it best we just start again with a new SE?

We used a structural engineer who was recommended by our builders. They had a relationship and it worked very well. At least if they know each other he will be more inclined to answer questions and help when needed. For a working SE this is a nuisance job in a lot of cases. Its a bit like working as a bathroom fitter/designer and then having to fix drippy taps. It sounds as though his assumption was based on the previous work being completed to a certain standard and maybe it wasn't. Did you receive any building control documents with regard to the work when you bought the house? We did not have existing steels when we took down a wall so ours was working with a clean slate so to speak. It sounds as though there are many factors at play here which complicates the job. You could go back to him and renegociate in light of the new discoveries. It seems a waste to pay for the same job twice. I would have expected a part drawing or sketch showing exactly where the steel was to be installed and padstone sizes etc. If you can do it without a lot of drama then I would go back to him and get him to revisit his calcs in light of what has been discovered and take it from there. If the relationship is now a bit broken rather find someone else. This is important you don't want to take any chances.