It's a good idea when the first flowers set, to use a liquid Tomato feed or, I use liquid sea weed once a week! particularly important for pot grown plants!Good luck!

Epsom salts will add magnesium (it's not really salt). I use a regular spoon (colher) per plant each month.

Liquid horse manure is rich in nitrates so is a big help but, as Paul says, don't use it too much until the first flowers set otherwise you will just get leggy plants. Epsom salts are a help certainly for magnesium but the important one is nitrogen. If you use a commercial tomato feed or liquid comfrey fertiliser it contains magnesium anyway.

Nitrogen is the worst thing you can give to tomatoes...too much and you get a lot of leaf, less fruit.
I would also be wary of adding to much magnesium(epsom salts) on a regular basis as that can affect calcium uptake leading to blossom end rot.
If you are growing in well prepared garden soil you won't need additional fertilisers during the growing season. Only grow bags and containers need fertilising with a proper tomato fertiliser. No matter what you grow them in, it's regular watering that gets the healthiest plants and the best crop.

Yes. Sorry Graham. I think I was having a senior moment, or, nowadays, a senior few months. High in phosphorous but low in nitrogen is far more sensible.