I wish everybody was concerned.
I lost 3 houses yesterday.
1 who cancelled outright, and 2 who clearly there's no point going back. 
And there may well be a few more yet Roger. The good news is that all the ones who decided to cancel did so in the first three or four months (except one who always was a moaner however I cleaned, so he beat me to it by a week). It got to the point where I was dreading the cheques coming through the post in case it was another cancellation or "final warning". I didn't go back to the "final warning" ones as I didn't want to give them the power (not very professional, eh?).
However, what was less obvious was the odd job that I was picking up - sometimes BECAUSE I had WFP and could reach previously inaccesible windows.
I lost about 5% of my old customers. About half of that was their choice - half of it mine.
In hindsight, some of the problems could have been prevented if I had gone around the first cleans twice. The problem I had was that, like you, I was under financial pressure and didn't want to do that because I couldn't really charge an existing customer extra for a first WFP clean. Therefore, I tried to make sure that I did an extra thorough job on the ones I really wanted to keep and took a gamble on the rest.
These days I have less customers but more work and that trend is gradually accelerating. I have a 6 week cycle that I now get around in 4. I turn over more money in the 4 than I did in the 6. I started a lovely job yesterday that I got because of WFP. I managed to do the lowest quote (because it's Georgian panes) and I work the way they want because it means that I can do it without treading on and breaking their tiles (like their previous window cleaner did). It should be a pound a minute job now I've done a thorough (and laborious) first clean - though I have to travel a bit.
N.B. In case anyone jumps down my throat for undercutting - their previous W/C had retired due to ill health and the slot was vacant.
WFP will sort out your customers.
Of all the jobs I lost, I can honestly say that I didn't miss one of them. Some of them are now paying double to a trad cleaner which shows me what I should have been charging them in the first place.