Counting the MEPs per English region is the easier case. Region is the linking field, and we can filter on a field from the secondary source (Party) with no problem, as we are using a simple aggregation, the count (CNT). For Scotland etc. the added complication is having to link on the devolved administration which we throw into the level of detail of the relevant 'Latitude (generated)' mark.
I mentioned that the count is a simple aggregation, other similar aggregations that 'tolerate' filters from secondary sources are SUM and AVG. Things get trickier when trying to look at parties, as we have to use the distinct count. We only want to count a party once in each region it occurs in, not once for every MEP.
min([party appeal])+(if max([party appeal])!=min([party appeal]) then ' & '+MAX([party appeal]) else '' end)MIN and MAX on strings is fine when there are only two options like in this case, so now instead of the hated '*' our devolved regions return 'UK wide & regional'. If we were dealing with a string field that had more than two values, MIN and MAX could still be useful to return an 'a-z' type range, if applicable. If it gets too complicated, maybe a blend is seriously limiting the analysis and another approach should be followed.