I wonder how much its work out per week for Jarvis and Lucy... i might give it a go if it's not a huge amount. 
Im not sure the feeding guildlines, and im also wondering about the high protein and also which one to order 
just realised the zooplus site has feeding reccomendations on there. Jarvis should need between 200-250g so a bag'll last 54 days according to my calculator.
Lucy only eats about a handful a day so i could feed her caviar and probably not notice much of a higher price!
So my Slabs will need alot more 
Slabs? Never heard of those before.

This is the chart.

Weight of Dog Active Dog Less Active Dog
1 - 10 kg 75 - 150 55 - 115
10 - 20 kg 150 - 275 115 - 200
20 - 35 kg 275 - 375 200 - 275
35 - 50 kg 375 - 500 275 - 400
50 - 65 kg 500 - 575 400 - 525
65 - 80 kg 575 - 675 525 - 690