Weekend first class upgrade but with split tickets, EMT

Good morning

This coming Saturday 27th July I am travelling from Leicester to London and return. I looked at split ticketing and can get two day returns (Leic to Kettering and Kettering to London). Total cost 45.78. I have a senior railcard which I presume brings that down to £30.52.

I was thinking of travelling first class. But.... EMT seem to have a weekend first class onboard offer of £12 for each way??

"At weekends you can upgrade to First Class on board our trains for just £12 each way, "
https://www.eastmidlandstrains.co.uk/travelling-with-us/first-class-train-travel/

My questions are:
1 Would I have to buy one of those £12 tickets for my journey from Leicester to London. Or would I have to buy two (L to K and K to Lon). Likewise on the way back. In other words would the upgrade cost me a total of £24 or £48?
2 Is there a cheaper way to get a first class return from Leicester to London?

Many thanks for any help

I'm 95% certain the upgrade would cover you for the whole of your journey anyway.

So if you do decide to split at Kettering which limits you to 1tph between Leicester and London, the total cost would be :

Leicester to Kettering Off Peak Day Return £9.05
Kettering to London St Pancras Super Off Peak Day Return £19.80
Total cost : £28.85 if you decide to upgrade to 1st Class both ways between Leicester and London this will make the total cost : £52.85

If i had the choice i would purchase the 1st Class Super Off Peak Day Travelcard between Leicester and London priced at £60.70, this allows you the use of all trains between Leicester and London, and gives you travel within London on the Tube, National Rail, London Overground and London Buses.

Many thanks sonic2009. Very kind of you to be so helpful and informative. Now I shall go and make a decision on whether to go for the split version with upgrade or the 1st CSOPDT (I used to be decisive but now I'm not so sure...).

Though a deciding factor might be how full weekend first gets on EMT? Does anyone know. If it gets completely full then I shall probably stay with Standard Class.

Separate question. Presumably using the split ticket method I can travel on any train on Saturday (travel at any time) as long as the train stops at Kettering.

You can travel at anytime with the split tickets at Kettering as there are no restrictions, infact the Super Off Peak Day return between Kettering and London is only valid at weekends only.

If you decide to take 1st Class between Leicester and London using the travelcard, it will offer you seat reservations the day before travel if you choose to book before hand and with some trains around 8am - 9am in the morning on the Saturday i'm sure you can obtain a free breakfast when travelling in 1st Class.

Many thanks again sonic2009. Very best wishes.