We use Marriages premix as well, mail order from SPR (I think?). We are going to try syringing Flubenvet powder mixed with Olive Oil because our chickens get very stressed locked in the run for 7 days with only pellets to eat, no treats. The only way they get the correct dosage.
1 scoop of Flubenvet treats 2Kg of feed which is enough for 2 LF chickens for 7 days. So one scoop with Olive Oill and one 14th of the resulting mixture (the Flubenvet adds about 0.8ml to the measure of oil) each every day. Then they can free range eating whatever they want.