I have just booked both crossings for £8.50.
I am going out via Eurotunnel and returning on a daytime ferry crossing from the Hook of Holland to Harwich, with a meal in the restaurant.
The trick is to cash-in your Tesco clubcard points, which isn’t real money, is It? You spend £5 worth of Clubcard points and you get £15 to spend with Eurotunnel. You get an equivalent deal with Stena Line.
The Eurotunnel trip costs £58 and I cashed in £20 worth of points to pay for this. Similarly, the Stena crossing from the Hook of Holland plus a three course meal cost £83.50 so I cashed-in £25 of points and paid £8.50 to settle their invoice.
Bargain.