anyone know of a decent laundry which washes with hot water either in the nimman or old city areas please?

You could inquire at Washy Mashy -- it's on Huey Kaew, beside Srithana school, on the Kad Suan Kaew side. It is not a self-service laundry. They have gas dryers, so they probably have hot water washers.

Are you sure you need hot water? The local detergents are formulated to work with "cold" water. I found a simple pre-soak with the local Breeze detergent before doing a wash cycle is sufficient to remove stains, odors, etc. I used to presoak some items at home before taking them to the laundry lady before we owned our own clothes washing machine.