I decided to take up the offer. After all, it was free money (yes, yes, I know the 'There's no such thing as a free lunch' adage). So, I took a look over the list of horses and decided to settle for the second favourite named Search For A Song.
The horse was 7/1. I went for £2.50 each-way rather than gambling on it to win straight up.
After placing the wager, I realised I had messed up by not ticking the 'Use Free Bet Promo' button. I had used real money instead.
It was quite early in the morning and I couldn't be hassled going through the process of trying to ask for the bet to be cancelled (I didn't like my chances), so I decided to take it on the chin.
I placed the free bet on the same horse and, to be honest, I forgot all about the mishap until later that evening.
I checked the results and saw that the horse finished in second place.
The return from the free bet was £3.50 and the comeback from the real bet was £6. So, I made £4.50 profit from the mistake.