These are Trangia fuel cannisters for use with a portable stove. They each contain 1 litre.
So that should increase the range of the bike by about 50 miles - or in other words since the standard tank is 4 litres this is now a 50% increase in the range for the bike...!
Now I have to test this using water first.
Also - the seat has gone away for repair. Thanks to DENNIS B. for arranging that.