I would use a light DME, any brand, any type, whatever is cheapest. No need to use Maris Otter extract, and I would steer away from LME and use DME instead. Reasons: LME has an extremely short shelf life, and will otherwise cause "extract twang" flavors which are far more expressive than any Maris Otter character that you could attain from any extract. Better off using any light DME, IMO.
If you want Maris Otter flavor, toast a couple ounces of real Maris Otter in your oven for 15 minutes, then crush that and steep it like tea in your brewing water before you add the extract. Bada-boom.
I emailed Muntons a short time ago and got a surprising response. It included "So, in answer to your question, I recommend that you use speciality malts such as light crystals and caras in order to get the full depth of flavour you would expect from an amber ale, instead of relying on the amber spraymalt that will give you colour but less of the fruity, crystalised sugar flavours."
I followed up with "I am especially interested in the recommendation to use liquid malt extract (LME) to get complexity of flavor. I am switching from LME to DME for all my brews because of the concern about LME staling, but maybe that is unnecessary. Is it your feeling that staling of LME is not actually a concern if it is used within a few months from receiving it?"
The answer: "You can usually expect LME to darken a little over time, which DME won’t. If the LME is fresh and has been stored correctly (cool conditions) then this shouldn’t be an issue, and it shouldn’t adversely affect the flavour."
I expect the difference between getting extract twang or not getting it is the age and storage conditions. Muntons seems to be confident that the LME would be fine, but I'm not so sure. Does the Muntons statement convince you, or do you still think LME should be avoided?