To find out which grains can be steeped, see Steeping Speciality Grains in John Palmer's "How to brew".
On a practical level, steeping and mashing are almost the same thing - you soak grains in water. The key differences between steeping and mashing is this:
- steeped grains have most of the starches already converted (Cara/Crystal malt, highly kilned malts) while mashing uses enzymes to convert starches in the grain to sugar
- steeping is done in 160°F/70°C water while mashing is done at around 148-154°F (mash temperature affects fermentability - lower temp = more fermentable.)
- steeping is done for 30 minutes, while mashing can be done for longer - one hour is typical.
In your case, while you can't technically steep the Maris Otter, since it's a base malt and the starches have not been converted, there's no reason you can't mash it, like this:
- Soak the crushed Maris Otter in 160°F water - about 1.5qt water per pound of grain - the combined grain/water should settle around 148-154°F
- After 30 mins, add additional water to bring the temperature up to 160°F
- Then add any other speciality malts (if any) and leave for another 30 minutes
- Optionally, rinse the grains in fresh 176°F water to get more sugars out
During the initial soak at 150°F the enzymes in the Maris Otter will break down the starch into the simpler sugars you want in the wort. Plus, these enzymes will also help break down unconverted starch in the other speciality malts that you would normally steep, giving you slightly better efficiency, as described in How does efficiency change when adding grains that are normally steeped to a mash?
A lot of fuss is made about mashing vs steeping, which seems to confuse the issue and scare away new brewers from doing mini-mashes. In practice it just involves different temperatures and longer time.