Are you sure about that? My understanding was that there is no protection in such a situation and any assistance would be purely discretionary and it's only if you hold a CIV ticket that you have a contractual right to assistance.
Where is the policy laid down?
EMT staff have been OK with it in my experience in the past (no idea if any new guidance given by EMR to staff) - but I did take the precaution of getting my eurostar ticket actually stamped by the train manager on the eurostar journey - I had to seek him out to ask for this to happen even though the train was very delayed, he had a special inking stamp to do it with when I asked - and was happy to stamp the ticket.
On the EMT train from ST P I got the impression the staff member was happy with my explanation and did not insist on seeing the Eurostar stamped ticket.
Obv this would be relevant for a Sheffield Journey.
I did make sure to set a 'reasonable' connection time between arrival and departs at ST P in case I needed to debate the matter. I would certainly not risk anything LESS than the set minimum time.
However I did think this was discretionary ultimately, but I may be wrong.
Frustratingly this situation can arise because Eurostar bookings open way ahead of UK bookings, which means through continental fares work to the latter deadline, by which the cheapest Eurostar only legs seem to sell out, driving up the price of the through ticket - or so it seemed to me, thus making me err on booking 2 seperate tickets, which would not have been my default choice.