From £40 (assuming a 100% match bonus) to £15,000 is some achievement! I would have thrown the towel in at £500 or so unless you had one single massive win that made your balance jump up by thousands by betting less than £5 a spin?
Whilst Vinylweatherman's posts are good and very informative, please don't put your hopes up too high by the very first words "This would be an unfair term".
You may well wish to take them to Court but what you're going to do when they offer a settlement of a few thousand out of court? Risk it all for a £20 investment?
It would certainly be an unfair term the way it's written because it has no floor limit, so one could end up with the ridiculous situation of never being able to play. This could happen if you received a £1 cashback as a no deposit free chip, and no game would allow a bet so small as 5% of £1. Whether or not such a situation might arise in practice does not matter, the term itself fails to cater for the possibility. Once a term is deemed unfair, it is struck out, and this would mean it's also struck out in other situations.
Another thing that makes such a term potentially unfair is that it sets a mathematical equation as one of the parameters for the player to follow, rather than a clear max bet limit.
If such terms were not seen as an issue, the CMA would not be running this investigation. They are doing so because so many players have been falling foul of these kind of convoluted rules for bonus play that the system of ADRs and UKGC can't cope with the workload.
Where the player probably has the advantage here is that if it went to court, the casino's defence would be along the lines that the player was betting more than £1 a spin with a 5 figure balance, and that this was deemed to be an "abuse" of the promotion, hence the term to prevent this was "fair". I can't see a judge accepting that bets of just over £1 with such a large balance is anything other than "fair play" given that the lottery is now £2 per selection because the government agreed that £1 is "too low" for a game that can be played by 16 year olds, 2 years less than the minimum age for playing in a casino.
Of course, it's no good simply stating "it's not fair" to the court, there would need to be a reasoned and logical argument as to WHY this is an unfair term in the context of consumer law and the kind of play that would be expected from an "ordinary recreational player" in the circumstances.
A consumer law solicitor would be able to advise on whether it's worth taking this to court, as the cost of doing so would have to be balanced with the likelihood of winning. If anything, waiting until the CMA have begun to give some indications of the types of terms it sees as "unfair", or even issued rulings outlawing certain types of term, would make it easier to assess the likelihood of having this term struck out as unfair by a judge.
It's also worth looking at other cases where consumers have succeeded in having terms struck out on the grounds of being "unfair" in other types of contract as it will give an idea of the kinds of things in terms that are getting struck out in actual cases. This is something one would expect one's solicitor to be doing when taking on such a case.