Maybe worth checking to see if this is something that your insurance covers?

I thought anti-siphon devices were fitted as standard these days.

Many petrol stations stock the business cards of mis-fuelling companies who come to you and sort it

You haven't told us the extent of the 'damage'. If the tank is still mostly diesel and you didn't add much petrol, you could probably fill up with diesel and continue as normal. If not, do as suggested above.

..... we put petrol into out 2016 Peugeot 308. Any tips on how to siphon it out please?

I have a siphon and an attachment allowing access to the fuel tank, but the 60cm pipe .....

You'll need some thin tubing considerably longer than 2ft!