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A quick 'back-of-envelope' calculation seems to support your theory. How difficult would it be to completely exclude all ambient light, or how easy to put a filter in front of the sensor, maybe coloured and chosen to let through light where the spectral responses of the LED and sensor overlap, with the intention of attenuating the wanted light less than the ambient light.

A filter is an interesting idea, I will look into it. I could probably cover the meter, but it's not an elegant solution that I would like to leave as a last resort. The meter has programmable relays which I think can do pulse output, but I'm struggling to find instructions on how to use and program the meter to do this. 

Have you seen this?

http://www.smartbuildingservices.com.au/docs/default-source/ProductPdfs/...

I've not downloaded it, it looks hopeful. But to sound a cautionary note, the meter communications might be encrypted so that you can't get in to reprogram anything.

[For info, the sum I did on your sensor was to convert the sensitivity (~ 1.5 V/( μW/cm² ) to ft-candles, and think how much light you get out of a candle a foot away - it's not much. It doesn't matter if the LED saturates the sensor, but you want daylight to give you as close to zero output as possible, and the bigger the difference between daylight and daylight + LED, the better. So you might need a neutral grey filter, possibly combined with a lens or shielding, as well as a coloured filter, to discriminate LED from daylight. You could watch the output as night falls and see when (if?) you get pulses - that would give you an idea for how much light you need to exclude.]

Those relays are typically to allow the provider to remotely control loads, for example my provider can turn my pool pump and hotwater element off if they need to lower peak demand.  Depending on the relay spec, they may just drive a contactor which in turn controls the load.    In some regions the end customer is allowed to override that (boost mode) by pressing some button, in which case the relay comes back on, but they're then charged at a higher tariff.

After a bit of digging i found this - https://de.wikipedia.org/wiki/S0-Schnittstelle  The meter has S0 output so i think i can do it. I am going to have to change my circuit slightly to make it work as I am using a raspberry pi at 3.5v and the S0 needs 5v min. 

Does the meter manual say you need 5 V? - it might not be absolutely necessary. The brochure I'm looking at says only 27 V, 27 mA max; no minimum voltage is specified.