Next episode in the water saga

I suspect this is going to be a long running story.
The current water basin holds something like 160l. The average daily (indoors) water usage is something like 250l (varies a lot depending what source you look at). Which would be fine if the inflow rate to the basin is fast enough.
We managed to run the basin dry recently and went to have a look at the inflow rate as we had not used the full 160l just prior to it going dry. We discovered that most of the water from the spring was running out down the side and not making it in to the basin, this meant that to fill the tank once it was dry was taking many hours.
To fix this we needed to have a look upstream of the basin, to do this we needed to excavate to find the channel. Having done this we found that some plant roots were in the channel and had blocked it by trapping small particles of mud. Once we scooped this out (gross and slimey) the water started flowing in to the basin again. We then needed to syphon the water out of the basin as it was all muddy from us digging around in the channel.

The water basin (with excavation above)

The excavation (it is deeper than it looks in the photo)

 We measured the inflow rate, after the recent very heavy rain it was approx 160l in 30 mins. We are not sure what the rate will be when it hasn't rained for a while. We need to wait and see.

We have spoken to the plumber about getting a bigger tank (rather than basin), probably 750l. This will allow us to use a bit more water in quick succession (eg a couple of showers and the washing machine) but it will not solve the problem if the inflow rate becomes much slower when it hasn't rained for a while.
This tank will also have a sensor in it like in toilet cistern but in reverse to detect when empty rather than when full. This would be a great improvement as the only way we have of telling whether the basin is empty at the moment is either going and looking at it (not practical during a shower for example) or when the tap runs dry - which means the pump is already running dry which will damage it if it runs for more than a very short period like that (very expensive mistake).
Ultimately we can connect to mains water as the next house down the valley has it (maybe 500m away), but it will be expensive to do as we have to pay for the channel to be dug and the pipe laid. If we did this we could continue to use the source water and only use mains when it ran out, as water is metered and (even) more expensive in France than the UK. Whatever happens having the source improved is a worth while investment.