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Sure, don't take someone else's view, have a look at the historic chart of the ftse or whatever index you want to track and see how often it is below the level of was x years ago.
Five feels a bit risky but I've not done the analysis.
Personally I feel the market is a bit toppy given the shit macroeconomic outlook and would be very wary of investing a lump sum now. But in not looking to do not done the work on it.
If you are going to need the money within less than 10 years then bank savings, premium bonds, etc make more sense than equities.
Reason is that stock markets have performed better in the long term but can, and do, crash by 20+% and it can take 10 years to get back to where you were beforehand.
You might be lucky and not see a crash while you are invested but it's a gamble.