How can I do the following:

At this moment in time I can not say I can recommend Opencart. But I am sure once I understand it better my view may change.

How do I set the following?

Scenario 1.
The customer puts 3 items (Lets call this 'Item A') in the basket - the total shipping for this should be £7. If the customer puts in 60, the shipping is still £7.

Scenario 2
The customer adds an item which has free delivery (Lets call this 'Item B') and adds some 'Item A'. So the total should be £7 WITHOUT the option of free delivery for the whole order.

Any help for the above is appreciated.

In Admin->System->Settings->Edit->Local
Set Length class to Centimeter and Weight Class to Kilogram and Save.
Create a Geo Zone if you need a new one for the areas that you will ship to in Admin->System->Localisation->Geo Zones
In Admin->Extensions->Shipping
Install and Enable - Weight Based Shipping
(Make sure that all other shipping methods are disabled.)
Select the Geo Zone that you have made or use an existing one eg. UK Shipping
Enter in the Rates for the selected Geo Zone: and make sure it is Enabled
All products that have Free shipping set to a weight of:
0.00000001
with a Weight Class of Kilogram and a length class of Centimeter
All other products set to their true weight or 0.011 kilogram whichever is the greater (otherwise just one of them will have free shipping applied) with a Weight Class of Kilogram and a length class of Centimeter.
You should find that both of your scenarios now work!
DO NOT ENABLE ANY OTHER SHIPPING METHODS!
If your unhappy with the text on the shipping estimate etc. would need to change following files:
catalog/view/theme/default/template/checkout/shipping_method.tpl
or:
catalog/view/theme/yourtheme/template/checkout/shipping_method.tpl
and also in:
catalog/language/english/checkout/checkout.php
and
catalog/language/english/shipping/weight.php
and maybe more!