Short update here. Finally my new ANT+ HRM arrived in the mail. I ordered a cheap unit from ebay for around $30 with shipping. It works great on both my Sony Xperia Active and a Samsung Galaxy S4.
I think the app will be much more stable catching the HRM devices now. It automatically pairs to the first device it gets a reading from. If you want to pair with another device, all youv'e got to do is clear the "HRM Device Id" field under "Settings / ANT+ Settings" and restart the App and it should pair with another device instead. Empty field is the trigger.
Hope it works as expected otherwise give me some feedback.
Happy training :-)