I have boiler cover, looking at the new tarrifs, it seems cheaper to take a new tarriff with boiler cover combined, if you already have boiler cover you can't have this tarriff?

Basically you need to sign up to both at the same time to get the discounted rate.

So generally to take advantage you would

have to cancel your current boiler cover and then sign up to your chosen Fix + boiler cover tariff. Or simply wait until your current BC is due to end.

This may help, but note that it is from 10 months ago so the T&Cs for the tariff may have changed and I would contact customer support to ask about your current options.

EDIT - Also note that the post following that one does suggest that it could be possible to do it without risking any cancellation fees (despite what those T&C say) - so long as you contacted support to do that for you.