Wey hey something I can answer!
Zebra an endowment mortgage is one type of interest-only mortgage. Other potential repayment vehicles are pension, ISA or straight forward savings. Basically anything that will give you a lump sum at the end of the term to repay the original capital you borrowed.
The only mortage guaranteed to repay your capital and interest is funnily enough a repayment mortgage - none of the other supply guarantees.
Endowments per se are not bad ideas the issue is where you have take a 'low-cost' endowment with a projected value suitable to clear loan but a guaranteed value of much less. You could take an endow with a guaranteed value of sufficient to clear loan but depending on circumstance this can be v.expensive.
Eddm - in your case you should have been supplied with a copy of the 'fact find' undertaken by the adviser and he/she should have been guided in product choice based on the information in that. If you can prove they have recommended the wrong produce then you can seek to have this rectified. For example they would have to cover all costs involved in redeeming this mortage.
On a practical level all I can suggest is you place some money into and ISA (remember you can both have an equity maxi-ISA and save upto to £14,000 per year based on current contribution levels (although due to change) but this has a degree of risk also as it is based on stocks and shares. Cash ISAs offer a fairly low rate of return and often better deals can be found in straight forward savings accounts, also contribution levels are lower. You could also try a unit or investment trust - again equity based with a risk level. And remember you can change mortage again as soon as your tie-in period is up. I a presuming you have one as if you didn't you could just change to a different deal as you wish.
it may also be worth approaching your mortage provider and explaining the situation - you mmay find they are helpful and will allow you to swap to a repayment product without penalty - as long as they are keeping your business they may be happy.
Please feel free to come back with any other questions or if I haven't made myself clear.
Cheers