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From the update:
Hello Everyone, It took me a while but I just worked out the amount needed to cover all the costs of the HMG and so it unlocks at £3,500. I am just checking my workings out for the Scrunt & Hatch and the 5 Prussian Scrunts, but I think they will all unlock at £4,464, will double check my maths first, plus I have to be really careful with the estimated casting costs and postage. Thanks for your patience Jackie :)