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So you've only told us half of the story - how much data do you want? Do you want an emonBase to send your data to emoncms.org to store and view it on the web, do you want an emonPi to store and view it locally on your LAN (and optionally send it to emoncms.org as well), or are you content with an emonGLCD and viewing live data only? Are you willing to shell out for another emonTx, or are you happy extending the cable from your current transformer? I don't think 20 m is prohibitive for a CT cable provided that there aren't serious sources of interference along the way. How easy is it to install that cable? [N.B. I get upset when English people use stupid foreign terminology for current transformers.] 20 m should be OK for the 433 MHz radio as long as you don't have thick walls and too many of them to go through.

I'm thinking of these possible arrangements:

1. You have an emonPi in the house, one CT on the PV infeed, the second on an extended cable to the grid infeed connection in your garage. The emonPi connects to your LAN/router by Ethernet cable or by WiFi.

1a. Like 1 but with an emonTx in your house, emonGLCD to view live data, no data is stored.

2. Like 1 but you have an emonTx in the garage reading the grid connection, sending data to the emonPi by 433 MHz radio (not WiFi). That opens the possibility of separately knowing consumption in the garage only.

3. Like 2 but with the emonTx and emonPi reversed, but then you need WiFi or an Ethernet cable from the garage to your router to get the pictures into your LAN. That gives you three spare inputs in the house to monitor separate circuits. If you're thinking of diverting surplus PV to heat water, that could be the preferred option using the emonTx to both monitor and divert the surplus PV.

Thanks for the response. 

So it sounds like it will work, it's a question of how.  Re: data I'd like more than just live data but am not bothered about having ooodles of info.

It looks like a good solution would be an emonPi and hope that the CT extension will work, with the backup that I have to use an emonTx to read the grid connection.  I would have to run the CT extension cable alongside the 25 sqmm cable that is the house electricity supply from the grid supply in the garage.  The supply runs in a 4 inch duct under a solid floor area between the garage and the house for 6m, other than that I can separate cables.  Not having a lot of experience of interference etc. I don't know if this counts as "a serious source".

I already have a consumer hot water surplus PV diverter - the immersion heater is fed from the garage so this was relatively easy.  I don't know if it will be better to replace this if I end up needing an emonTx?

I think if you use good quality screened cable, you might be OK on the interference question - it's a black art and you'll only know after it's set up and installed. Ideally, I wouldn't have run it alongside the main feed at all, but it seems you have little choice. Previous users have reported problems with faulty fluorescent tubes and PIR-controlled lights, but it often turns out they used unscreened cable. I've posted many times the correct way to do it, I must write a Building Blocks page.

If your existing diverter is performing correctly, there's no point in replacing it. But some users have found that some commercial units don't balance accurately and allow a small 'bleed' of energy back into the supply. I think you'll find that out when everything is set up and working. The burst-mode diverters described here rely on your meter having an energy 'bucket' inside which energy can be bounced backwards and forwards into the grid without being charged, so you'd need to establish that before making a decision.

In terms of the data, you'll need to play with emonCMS, or look at the user dashboards that have been published, to see what you can have. Unfortunately, none of this has any "glossy sales brochure" type of documentation.