Advanced Fares and travel to origin

I have an Advance ticket for the 0730 Euston to Glasgow service on Saturday morning. I'll buy a ticket to St Pancras on the day and walk the short journey to Euston. If my journey to St Pancras is delayed and I miss the 0730 will I still be able to board a later service? I won't actually have ticket to Euston.

Only if your train into St Pancras is timetable to arrive at or before 0655 (i.e. 35 minutes before 0730, derived from 15 min interchange at Euston + 15 min interchange at St Pancras + 5 min connection)

There will be the obvious (not necessarily correct) argument that you are unticketed between St Pancras and Euston, despite it being walking distance.

You will have been expected to leave a reasonable amount of time from your train arriving at St Pancras to the 0730 departure from Euston. If you train is timetabled to arrive at St Pancras at 07:16, you will be asked to buy a new ticket even if the train you were on is delayed as you would not have left sufficient time to make the connection regardless of if you are tubing or walking it.
As was said before, if your train is timetabled to arrive at St Pancras before 06:55 and you still miss the 07:30 through no fault of your own, the staff will endorse your ticket for travel on the next service. If this does happen, be sure to keep the ticket you used to get to St Pancras as proof that you were delayed by a national rail service.
Try to get to St Pancras for about 06:45 to be on the safe side.

bcarmicle said:

Only if your train into St Pancras is timetable to arrive at or before 0655 (i.e. 35 minutes before 0730, derived from 15 min interchange at Euston + 15 min interchange at St Pancras + 5 min connection)

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Thanks, all, very helpful replies.
I can get a train that arrives at 0647.

Ticket Man said:

if your train is timetabled to arrive at St Pancras before 06:55 and you still miss the 07:30 through no fault of your own, the staff will endorse your ticket for travel on the next service.

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If the OP is walking between the two stations, I don't think it can be taken as certain that the ticket will be endorsed. It probably should be but it's Euston we're talking about.

Haywain said:

If the OP is walking between the two stations, I don't think it can be taken as certain that the ticket will be endorsed. It probably should be but it's Euston we're talking about.

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This can be solved by buying a ticket to London zone 1 or an outboundary Travelcard, then the tickets overlap. The staff at Euston have no way to know if you actually did use the Tube. Will cost an extra 4 quid though.

I would then use a journey planner entering the through journey to work out which train into St Pancras you use. Things are always easier with an itinerary.

Bletchleyite said:

This can be solved by buying a ticket to London zone 1 or an outboundary Travelcard, then the tickets overlap. The staff at Euston have no way to know if you actually did use the Tube. Will cost an extra 4 quid though.

I would then use a journey planner entering the through journey to work out which train into St Pancras you use. Things are always easier with an itinerary.

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Yes, it's only a few quid extra, and I'll have the options of staying underground if Storm Dennis hits early!

Many journeys via London involve walk or bus/tube from one station to another. Surely the normal rules about split ticketing apply?