Thank you everyone for your comments.  I have found some 1N5822 Schottky diodes 
so may try those.

Chris: 1N4148 - yes please.

John F5VLF

On 11 Jan 2011, at 12:57CET, Julian Hardstone wrote:

> In that quantity the 4148 costs less than one pence, so before you go buying 
> stamps let John pull a couple out of any scrap pcb he has lying about - 
> there's always some small glass diodes which would serve
> - Julian G3TFR
> 
> From: "Chris Trayner" <[email protected]>
> 
> 
> Hi John,
> 
>> Not having any 1N4148 diodes I used 1N4007s.  Might this be the cause of the 
>> problem?
> 
> I have a box of 5000 1N4148s at work. I might be able to spare you the odd 
> one or two ;-)
> 
> 73,
> Chris G4OKW 
> 
>