SSH: Key refused

I am having an issue using certs via Pageant to login.
Not a killer issue as I can just use Putty for sites that don't work. (however it would be handy if it did work)
The exact same key works directly in Putty and for this example I've stripped all other keys out so only the key concerned remains.
example :

[quote]Opening Connection domain:22
Server version: SSH-2.0-OpenSSH_4.3
We claim version: SSH-2.0-PuTTY-FZ-Local: Sep 19 2012 11:46:21
Using SSH protocol version 2
Doing Diffie-Hellman group exchange
Doing Diffie-Hellman key exchange
Host key fingerprint is:
ssh-rsa 2048 xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx
Initialised AES-256 client->server encryption
Initialised AES-256 server->client encryption
Pageant is running. Requesting keys.
Pageant has 1 SSH2 keys
Trying Pageant key #0
Key refused
No supported authentications offered. Disconnecting
SSH: ssh_init: error during SSH connection setup -1
Connection closed[/quote]

I think this is similar to this post:

The ftp(s) and sftp feature of Dopus are absolutely brilliant (and essential!) - but some polishing here and there might be needed?

Oh dear...
I just realized what the issue is: the key concerned is passphrase protected.
Using a bare key works OK. (which I already knew)
Sorry about the noise.