Waterproof coat for muddy environment - small or medium (Claregate WV6)

Hi, I wonder if anyone has a waterproof coat lurking in their cupboard, perhaps the type acquired as workwear but no longer needed. Scruffy is fine, as long as it is waterproof. I am a horticulture student. My current waterproof, despite washing with Nikwax, is not keeping me dry when it rains. I'm reluctant to buy a new coat (although my husband has kindly offered to get me one for my birthday!) as in the environment where I do my work experience I am using power tools, getting splattered with mud, hugging plant puts etc, so it seems a shame to buy something new and expensive. I usually buy ladies' size 12 coats, so anything small to medium would fit me.