5p is just the nominal value of each share. It doesn't usually mean much more than the value when the shares were first traded - it's a bit like saying that a Penny Black has a value of a penny. When the market closed today, each share was changing hands at £7.10 (the share price is given in the top right corner of most of their web pages). The small difference between 1435 x the old share price and 1018 x the current share price plus the value of your cheque is just due to a small change in the market value of the company.
I am puzzled by why the nominal value changed from £5 to 5p but this does not have anything to do with the value of the company. You still "control" the same percentage of the company as you did before but, instead of owning a percentage of a business plus a large pile of cash, you now own a percentage of a business plus a small pile of cash - the difference between the piles of cash has been given to shareholders like yourself.
You have not been scammed and your investment is as safe as any other investment in the larger UK companies, perhaps moreso.