Hi, I'm new to modelling and wondered what others could recommend as a suitable 2-part epoxy putty to use for figure alterations and making tarps etc. Thanks

Miliput and Apoxie Sculpt come quickly to mind.

I use apoxie clay and apoxy sculpt almost exclusively for everything from tarps, filling gaps, reworking edges, weld seams, etc. I have not come across anything I like better

The softer, dough-like putties are good for general sculpting (Milliputt, Apoxie Sculpt, etc). The denser putties, with a "chewing gum" style consistency (Tamiya, Knead-a-Tite), can be better for sculpting fine, sharp details, like patch pockets, epaulettes, and belt buckles. These can of course, be used together, as needed.
Steve Zaloga's modeling books (published by Osprey) have tutorials with photos, showing how to make tarps and mantlet covers with epoxy. His "Modeling US Armor of World War Two" is available discounted through Amazon.com (buy from Amazon itself, as some of their affiliated dealers have jacked up the price).