They seem to be coming along OK in my two London ponds, though I did
see one fish snatch a tadpole. Have seen some medium size newts, which
probably came out of hibernation somewhere, but the tadpoles seem to
be taking forever to develop.

I have read this, can it be true?

"The length of frog development from egg to tadpole to frog usually
takes between 6 to 12 weeks.
But it is also temperature dependant, so during cold spells it may
take a bit longer or even be suspended till the temperatures go up.
For example, eggs laid towards the end of summer may hatch, but
tadpoles may stay tadpoles until the Spring/Summer period. So if it's
cold and your tadpoles don't seem to be growing up very fast, it's no
reason to panic."

http://allaboutfrogs.org/info/tadpoles/index.html

Its fairly cold just now.

Anyway, I have had to stop using my fountain to avoid tadpoles being
sucked into the filter but the oxygenating pond plants are coming
along nicely so may be providing enough oxygen for the time being, or
else must I buy a bubble oxygenator?

Doug.