At 50 nights you'll be granted the Annual Choice Benefit which includes the option for 5 additional nights. 55 nights + 5 additional takes you a little closer, to 60, just 15 short. If you're unwilling to book some cheap nights, you could try opening a Marriott credit card that grants Marriott nights, e.g: the "Marriott Bonvoy Boundless® Credit Card from Chase" grants 15 elite night credits, as does the "Marriott Bonvoy Bevy® American Express® Card" (and the "Marriott Bonvoy Brilliant® American Express® Card" offers 25 but with a much higher annual fee).

edit: ah! You're British. My condolences. You're limited to just one option, the "Marriott Bonvoy® American Express® Credit Card".