Yes, each plant is available in 3 different sizes:

Plug
9cm Pot
1 Litre Pot

There isn't a "plug set" as you describe it. Plants are purchased as a single product but can be either a Plug, 9cm Pot or 1 Litre Pot.

We want to apply a discount for the following bulk purchasing:
0-14 plants
15-29 plants
30-49 plants
50-74 plants
75-99 plants
100+ plants

So, in the example you've given, there wouldn't be a price difference, per plant, between 5 plants (plug size) and 10 plants (plug size) because they are both in the first discount range (0-14 plants).

However, the pricing, and discount rate, changes depending on the size you've chosen.

Here is a full breakdown of the pricing and discount rates that we're trying to acheive.

Plugs:
0-14 plants = £1.15 each
15-29 plants = £1.00
30-49 plants = £0.98
50-74 plants = £0.96
75-99 plants = £0.94
100+ plants = £0.92

9cm Pots:
0-14 plants = £1.75
15-29 plants = £1.60
30-49 plants = £1.54
50-74 plants = £1.52
75-99 plants = £1.50
100+ plants = £1.48

1 Litre Pots:
0-14 plants = £3.50
15-29 plants = £3.20
30-49 plants = £3.15
50-74 plants = £3.10
75-99 plants = £3.05
100+ plants = £3.00

I hope that makes sense. Appreciate the help Craig, thanks.
Nath.