Right, DH has spoken to the engineer a couple of times. He has re-worked the calculations but says the mistakes make no difference to the recommendation. It seems to me he's still not calculating it properly so I've now sent him this which covers all of the brilliant advice you've given above! If he doesn't change his mind about it all at the end of this then I think it proves he would recommend underpinning the wall before hanging a picture on it 
in which case we'll have to find an[other] engineer.
^ " Load assessment / recommendation
1 when I spoke to you earlier you said that you had taken account of the piers on our side by placing 25% of the load on the party wall but doing this doesn’t take account of the piers. 50% of the load should be taken to be on the back wall of the main part of the house, with 25% falling on the pier on the East wall of the kitchen and 25% falling on the pier on the party wall side of the kitchen. If you place 25% on the party wall you take no account of the load carried by the piers on which the beam rests. If any of the load is taken to fall on the party wall it should presumably be much less than 25%. If the calculation is corrected to reflect this (and points 2 and 3 below) it makes the increased load on the party wall from the loft conversion either nothing or next to nothing;
2 per my previous email – the calculations for our neighbour’s joists do not reflect that the joists run from front to back rather than side to side;
3 per my previous email – the DL figure is incorrect;
4 the foundations have already been proven for a higher load: the foundations to the party wall in the main part of the house must carry a greater load than the load that the party wall in the kitchen would carry after the loft conversion. This is because the party wall is taller (circa 8m), the ground floor is a suspended timber floor and the stairs run up that wall;
5 my understanding is that it is very unusual to underpin for a loft conversion. The percentage increase on the party wall involved here is very small and next to non-existent if the joists are run from front to back. In those circumstances an underpinning suggestion is difficult to understand;
Assessment of bearing pressure
6 as you’ve said, you’ve missed the section of the party wall in the loft. With this taken into account the existing load on the party wall in the kitchen must be at least 30 kN/m2 and in the main part of the house is greater still. As the building has stood in its present position with no problems for 115 years the bearing pressure cannot be 50 kN/m2 (24.5 kN/m run allowable line load);
7 having read up and taken some advice on this, I understand that the 50 kN/m2 bearing pressure you have given the underside of the existing foundations is extremely low. The description I was given is that the ground would be “as soft as butter”. This seems vastly different to the “hard and stony” description that the building control department observed when inspecting the ground for our neighbour’s extension. I was also advised that if the ground is really that soft the house would have subsided already (see point 5 above);
8 your assessment of capacity describes the bearing pressure as “soft/medium clay”. Is this a mistake or is your assessment that the ground here is soft/medium clay?
9 I am concerned that you have not tested the soil in any way as my understanding is that this should be completed if there are concerns about it. Do you have a geotechnical engineer at [name of company] that could be consulted?
We would like you to re-run your load assessment to reflect the points above and to re-consider your conclusion in light of the corrected assessment and points 4 and 5.
Please also re-check your interpretation of the soil bearing capacity. If it would help, we can re-excavate the trial pit to give you the opportunity to re-inspect the conditions, perhaps examining the soil conditions below the base layer, perhaps using some basic soil strength testing equipment and perhaps carrying out the assessment with your geotechnical engineer if you have one in-house. We will call out building control to complete their assessment too. [...]
You mentioned it today and we fully appreciate your need to be conservative in your assessment but clearly there is a balance to be struck. Ultimately we have instructed you to produce a fair and balanced report with appropriate and safe solutions that meet our needs. I am sure you understand that an excessively conservative assessment that will cause us unnecessary work costing in the region of £10k (and months of further delay) is of no value to us.^ "