Gas suppliers

[Busy Bee]

We have just contacted GDF to connect our gas supply after the plumber has finally finished - well the important bits anyway!  And GDF have told us to choose and contact a supplier, who will then contact them to connect the gas.  I have found the 4 suppliers but they seem super complicated - does anyone have any recommendations, or is there a comparison website I can look at - is there anything in particular I should be looking for?  It's for an apartment to let, so the tenants will be paying it, but I'd like to get the best deal for them - we need to get it connected to make sure it's all working correctly

Thanks to anyone who can help.