Use highest price shipping when different classes in basket

I’m using shipping classes and have set up my shipping so that for a particular weight range there are 3 postage costs (3 different classes). This is working perfectly when there is only one product class in the basket, but when I add products from various classes the postage is either picking the lowest price for that category, or adding the price for 2 classes together. I need it to pick the highest price for the relevant classes that are in the basket.

In the UK postage is calculated on weight and dimension. So a flat light item could ship at one price and bigger, but still light items would ship at higher prices (small and medium parcel prices).

I have set up a base price for Uncategorised shipping classes (most of my products) at ￡3.20 for items between 0-0.25kg.

I have set up LETTER at -￡1.67 from the base price (as postage is ￡1.53)
I have set up MEDIUM at ￡1.9 from the base price (as postage is ￡5.10).

I would like the postage for:-
LETTER + Uncategorised in basket = ￡3.20
Uncategorised + MEDIUM in basket = ￡5.10
LETTER + MEDIUM in basket = ￡5.10
LETTER + Uncategorised + MEDIUM in basket = ￡5.10
i.e. always pick the highest price for the relevant classes in basket.

What I am getting is:-
LETTER + Uncategorised in basket = ￡1.53 i.e. lowest price
LETTER + MEDIUM in basket = ￡6.63 i.e. adding together the postage prices.

I have also tried setting this up so that ￡1.53 is my base price and having a new shipping class – STANDARD at +￡1.67 and MEDIUM at +￡3.57 but this doesn’t work either.

Is it possible to set this plugin so it picks the highest postage of the classes in the basket? And if so, how?

Thanks in advance for your help!