Kurt, BKS beat me to one comment I was going to make.
There are several other comments I was going to make.
Firstly, the error message you get on the BA site most likely relates to two points. These are that BA do not fly to Kathmandu and flights are not bookable on their website more than 355 days in advance. Other airlines have their own date restrictions.
Secondly, The times you mention do not match up with those mentioned for BA MAN to DEL flights the week before. Also, make sure you allow plenty of time between arrival at LHR and departure to DEL. Your suggested schedule looks a little tight and allows little/no leeway in the event that your first flight is delayed.
Thirdly, obviously, I don't know which is more important to you, cost or flying BA MAN to DEL. For flights in late October 2013, BA are quoting over £1300 one way. Around that time the airline which is an anagram of Teahid will do MAN to Kathmandu via Abu Dhabi for a little over half that. Expedia, with cost in mind, only uses BA to LHR then Jet to DEL and beyond.
Obviously your money your choice. You might, in a day or two when BA flights become bookable, search the trip on Amadeus to get some other options.
Whatever you decide, enjoy the trip.
SFS