Помогите новичку! Первый в жизни серьезный документ! (Юридич) law

[S.Pero]

Всем привет! Помогите, пожалуйста, адекватно отобразить смысл и стиль вот такой странички. Впервые в жизни перевожу контракт, очень боюсь подвести клиента:

By this letter we, the undersigned, ...................................................... , a Bank (or company) organised under the laws of ................................................... and having its registered/principal office at ............................................... do hereby jointly and severally with the Contractor irrevocably guarantee repayment of the said amounts upon the first demand of the Employer and/or the Northern Dimension Environmental Programme without cavil or argument in the event that the Contractor fails to commence or fulfil its obligations under the terms of the said Contract and in the event of such failure, refuses to repay all or part (as the case may be) of the said advance payment to the Employer and/or the Northern Dimension Environmental Programme.
Provided always that the Bank’s obligation shall be limited to an amount equal to the outstanding balance of the advance payment, taking into account such amounts which have been repaid by the Contractor from time to time in accordance with the terms of payment of the said Contract as evidenced by appropriate payment certificates.
This Guarantee shall remain in full force from the date upon which the said advance payment is received by the Contractor until the date upon which the Contractor has fully repaid the amount so advanced to the Employer in accordance with the terms of the Contract. At the time at which the outstanding amount is NIL, this Guarantee shall become null and void, whether the original is returned to us or not.
Any claims to be made under this Guarantee must be received by the Bank during its period of validity.

Мои несчастные попытки на этот счет таковы:
Данным документом мы, нижеподписавшиеся, ………………………………………………………………………………, Банк (или компания), созданная согласно законам…………………………………………., и имеющая официальный зарегистрированный офис в………………………………………………………………………………………, совместно с Подрядчиком и индивидуально гарантируем безвозвратную выплату оговоренных сумм по первому письменному требованию Заказчика и/или NDEP, без претензий если Подрядчик не выполнит свои обязательства согласно условиям контракта и если, с случае такого невыполнения обязательств, Подрядчик отказывается вернуть предоплату (или ее часть) Заказчику и/или NDEP.

Остальные два абзаца вообще порядком меня озадачили...
Не дайте погибнуть начинающему переводчику! Заранее ОГРОМНОЕ СПАСИБО!