It depends how the interest is added ie, whether they add the interest on as you go along or just once a year, adding 5% to the amount you've saved.
What you need to be very aware of is how much it will be worth in today's terms. Surprisingly with only a 4% inflation rate money roughly halves in value over 20 years so your savings are only going to be worth around £20,000 in today's terms if inflation is at 4%. Of course it's impossible to predict how inflation rates will vary over 20 years, I'm just trying to point out that even a fairly small inflation rate has a marked effect on the worth of your savings over 20 years.