Lifted from t'internet...
"Error code E13 like the dreaded number can be unlucky for some. Fault E13 indicates that the main heat exchanger thermal fuse has blown or failed and I am sorry to say is a catastrophic failure meaning the main heat exchanger will require replacement. That said we would often advise our customer to do a straight boiler swop as it would be quicker and may be cheaper in the long run as the main heat exchanger is a very expensive item. This is a rare fault and is normally caused by external faults or running the boiler whilst dry. You maybe lucky though and find the fault is a wire problem between the thermal fuse and the PCB. Pump checks and boiler pressure should all be checked but the F13 Fault code is normally serious. The F13 is thankfully a very rare occurrence on the generally well behave Remeha Avanta series of gas boilers. "
Sounds a bit moody sorry to say...
I had fitted a Baxi Platinum 33 in March of this year and it's been really good with a great hot water rise and flow rate, fills a deep P shaped shower bath in around 6 minutes, just add cold to suit.
The heating in my modest semi, with 10 radiators is piping hot in a short time.
It's a minor step up from the usual 28-30kW options and needed a 28mm gas feed rather than 22mm but that was no biggie in my situation.
I'd highly recommend it.