Whilst you have had some bad experiences with your property letting it isn't always the case. You can reduce your risk in a number of ways. Perhaps sell your house and buy smaller houses in the order £100K. This spreads the risk. With property agents it can be a lottery as to how good they are.
There are agencies that buy property and offer a full letting service on your behalf even abroad. I would suggest you further minimise your risk by not furnishing the house and not buying expensive fittings - you can afford less attractive fittings with a cheaper house.
I echo previous comments made in this thread that you have to remain detached in your dealings. Property letting over a long term does not suffer as many vagaries as the stock market but the downside is that there are potential pitfalls as you have found.
I am only drawing on my experience and I have experienced a number of hiccups but none as bad as yours and my letting agent to date has been quite good.
HTH
GB2