I agree with the above with regards layering. I often use a lighter weight sleeping bag and either line it with an appropriate liner or use a gortex bivi bag. The liner option is a good one as it means you can wash that rather than the bag. The bivi bag also works well as it adds a 'season' and packs down well but also enables me to sleep rough if the weather is good enough but at the same time keeps me dry if it does shower.

Regarding your size, well thats a bit more of an issue. Most mummy bags are, by definition, uncomfortable as they make you sleep in an unatural position and are not built with larger individuals in mind. One option that might be worth considering is a cheap cotton square sided bag. I use one a lot. Its cotton, so more comfortable and not sweaty. Its wide enough for even Harley riders and still packs reasonably small. So for your 80 quid you could easily get a two or three season cotton 'caravan' bag with ample width and have change for either a liner or a cheap breathable bivi bag. This would give you the flexibility and comfort that we all seek. Try and avoid the flowery ones though as you will just get laughed at!

Mummy bags are great for use in tougher condition and I have several for use in snow holes, high mountains and sea kayaking but when I am on the bike I use the cotton option above. One cautionary note though. Cotton looses thermal values when wet. Thats why most mummy bags are made from a non absorbant artificial material.

One more thought. Recently I took a group of young sea kayakers off to France and one of the kids had a German Army bag that had arms! I had never seen one before but might be worth a hunt around. It was very amusing as he wore it as a coat of an evening as it also flipped up below his knees to enable him to walk. So when bedtime came, he just lowered the leg coverings and lay down where he stood. That would solve your arm problem.

Hope this helps a little but ultimately its a very individual thing.

__________________
DickyC

Gun Metal Grey 2014 XT660Z
'Advice is sought by someone who knows the answer but doesn't like it'

'The adsence of alternatives clears the mind marvelously'