booking in train tickets in advance

What exactly are you trying to book? The pass is your ticket.

If you want to buy a mobile pass, then you don't need a passport number yet. You may want to wait a bit to buy your Interrail though, since there's usually a sale sometime in March.

We’ve got our interrail passes number but needs passenger information to book the eurostar train journey

Ah you mean a Eurostar reservation.

You can indeed change your data later, on the Eurostar website. The data is not for the reservation, but IIRC for the UK government.

Hi, thank you for your help and answers;

We are booking these eurostar tickets via the interrail site using our interrail pass.

There is no option to add passenger details later.

Eurostar does let you enter the details later but this is not the case on the interrail site...

I've no idea why Interrail wants your passport number to make a reservation. But you're right, I seen to remember having to put my passport details in when reserving via their service.

It's not a piece of information that they need to pass on to a train company, with the possible exception of Eurostar - and even with Eurostar, you can change it on their website later. 
(And recently, many people's passport details were compromised in a hack.)

In any case, you can put your current details in if the Interrail site demands a passport number. 

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The Rail Planner app, On the other hand, does need your correct passport or ID when activating a pass