Ellen /ɛlən/ (with variant Helen) is pronounced with an /ɛ/. This is the vowel found in SET and FELL. Alan /ælən/ (with variants Allan and Allen) with an /æ/. This is the vowel found in CAT and PAL. 

But the vowels /ɛ/ and /æ/ are admittedly fairly close in sound, and can be difficult to differentiate for those who don't use both vowels in speech. This may be where your confusion lies.  As you can see on this general American vowel chart, they are very close to each other on the lower left.



EDIT: To the specific point in your question, "Being a Brit also, the names 'Ellen' and 'Alan' tend to sound the same when spoken with an American tongue" - well, that IMDB commenter from somewhere in the British Islands (an area with hundreds of dialects) may not hear the difference - but Americans certainly pronounce the difference.

Do Americans pronounce "Ellen" and "Alan" in the same way?
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