Hi Hannah, yes it is the 20053.It sounds like your batterys are on the way out. I suspected mine was acting up due to the batterys(they only last 12 months,probably sat in a warehouse for a good deal of that) one in the strap and one on the watch/monitor,so decided to change them(both),its very straight forward. Fingers crossed it seems to be running ok now,although its only been a day with the new batterys. Give it a whirl dont buy batterys from newsagents way to expensive,bought 2 off ebay free p+p £1.49. heres the link.
I bought one of these, set it all up. I have used it twice and seemed ok but its started acting up, losing all my input settings,even the time. It has happened twice now and i suspect maybe its the battery,but its not even a week old.
In reply to madmo, have you tried wetting the contact strip slightly on the chest strap.I found this helped although it wouldnt give me a reading when used on my bike with the handlebar attachment.
I regret buying it so i'd say give it a wide berth.