Hi @sarahg with 2 easy changes you can reduce the reservation cost down to 20€  with bad luck 40€ 
Use anytrain from Zürich to Basel and then a Regionaltrain to Mulhouse or Strassbourg (reservation free) 
From either Mulhouse (Gare du Lyon) or Strassbourg (Gare de Est) you can then use the cheap domestic Reservations and book them via B-Europe online Gare du Lyon and Gare de Est are the arrival stations in Paris :) 

Do not book that train from any place in Switzerland.

Got to Mulhouse in France with regional trains, where you'll be able to reserve the same train for 10 (or 20) EUR per seat. 

Ok -  Thanks - off to check the double check timetables but I think we might need the high speed to make our connection back to the uk and embarrassingly there is strong desire from my son (and maybe me) for a double decker train each trip.

But off to do due diligence on the timetable with your advice, thanks

You'll be able to travel in the fancy double deck TGV Lyria, but from Mulhouse. You'll just need to leave a bit earlier from Zürich and you'll save A LOT of money as TGV Lyria has extortionate reservation fees from/to Switzerland.

Shouldn't be a problem to get to the UK. 

Or are you starting your return journey somewhere else than Zürich in that day? 

Ahh I’ve been rumbled...yup this is the last monster leg home at the end of our trip with a full day of train travel for which I fear I may be judged, so mea culpa, I have not shared the full details.

Ok so we’ll wake up in St Mortiz on the morning of the 10th and we're on the 20:13 eurostar from Paris, thus with check in, crossing Paris and stuffing some food in our mouths I figured we need to be in Paris by at least 6:30pm, earlier better but we could just grab a burger.

I really want to do the 9:02 st Moritz to Chur (arr 11:16) so assuming enough time either at Chur or somewhere around lunchtime to grab a coffee and delicious pastry lunch (to go) what routes do you recommend to do Chur to Paris in 6 or so hours, bonus if any are double decker trains! 

My son is 13 so short interchanges not a problem but regular food input is essential.

Thanks

The Strasbourg to  Paris TGV comes into Gare d’Est so no crossing Paris needed as it’s a 10 min walk to Gare de Nord for Eurostar

The timetable I see on SBB has your train arriving in Chur at 1104, not 1116. That allows a connection onto the 1108 IC to Basel. It’s a short, but legal 4 minute connection in Chur.

Or you could get the earlier train from St Moritz 0851-1055, which is the Glacier Express. There’s a supplement to pay on this, but it would give you a bit more time at Chur, maybe even enough to visit the Co-op.

From Basel, you take the 1420 TER to Strasbourg (no reservation needed), and then the 1619 ICE, or 1719 TGV to Paris Est. Both would give you enough time to walk 10 min to Gare du Nord. There’s a chance you could make the 1544 to Paris, but I think that wouldn’t be a legal connection.

Or the same 1420 to Strasbourg also calls at Mulhouse, where you could pick up the TGV-Lyria to Gare de Lyon. Although that gets you into Paris a bit earlier, you lose that time again getting to Gare du Nord.

Edit: this doesn’t allow you to stop for food until Basel, where you have time for lunch. But you have a restaurant onboard the IC from Chur (at eye-watering prices) or stop off at a Co-op on the way to St Moritz in the morning!

It is an entirely doable journey, don't worry.

I'd avoid the TGV Lyria fees from Switzerland too.

- St. Moritz - Chur 09:02 - 11:04

The change is at the same platform, it is a guaranteed connection. If for some reason (very rare) you are delayed by more than 10 min, there will still be later connections before your TGV and Eurostar

- Chur - Basel SBB 11:08 - 13:28

- Basel SBB - Strasbourg 14:20 - 15:39

- Strasbourg - Paris-Est 16:19 - 18:05 (not double decker) / 17:19 - 19:05 (double decker)

As already said Paris-Est is better to arrive as it is about 10 min walk to Gare du Nord. No need to take any RERs or subways.

- Paris-Nord - London St. Pancras 20:13 - 21:30

Edit : or you leave St. Moritz at 08:02 and can make the 15:44 from Strasbourg. Of course you then have 2h30 in Paris

Strasbourg - Paris-Est or Mulhouse - Paris-Gare de Lyon is already available here btw :)

https://travel.b-europe.com/Eurail-GE/en/booking-tgv#TravelWish

Thank you all so much - Paris-Est! of course - *slaps hand to head*

Maybe we do glacier express down from St Mortiz - right to the timetables

Do not take the Glacier Express. The 42 CHF seat reservation per person won't be worth it.

Use the 09:02 IR train as you wanted and 99% of the time the connection in Chur will work. It is on the same platform, I've done it a dozen times. If something happens, you will have later trains to Strasbourg. Don't worry (and don't pay 84 CHF...)

Right, a glass of wine, timetable and train map all concur it will be a 6 train 13 hour jaunt back to the uk (inc uk train home)

I shall not be seduced by the Glacier express and its fancy glass roof but stick with our planned train armed with Swiss breakfast-y snacks.

Chur → Basel  then an hour to hit a bakery for boy and coffee for me

Basel → Strasbourg then just under 2 hours to eat a good meal (he remembers a bao place we found there last year) cos of course we gotta do the double decker to Paris Est, grab a few bags of Brets crisps and a cold beer for me on the way to Gare du Nord and home.

Many Thanks everyone for input, much appreciated.