Product replacement or discontinuation is obviously quite usual - but that doesn't alter warranty obligations. The manufacturer should have stocked sufficient to meet likely needs. Without knowing the value. It is hard to know whether it is worth pursuing them; right now it sounds like they are ignoring you in the hope you just give up.

Hi Tim,

Thanks for your viewpoint and I couldn't agree more with you on this.

However, they have actually discontinued the product and their revised e-catalogue confirms this. I am sure they understand that a Purchase Order from us would just add on to their revenue in multiples at best, to risk such ignorance. Of the 28 product categories they have supplied, the issue affects only 1 of the 28.

The query deals around with
(a) non availability of spare parts of the product and
(b) non availability of the product itself.

Experience suggests this will boil down to the terminology used within your warranty document. It may be unreasonable to expect a supplier to maintain full stocks for all warranty potential on discontinued products (Tim already mentioned value...). More commonly, suppliers offer repair or replacement with the direct alternative product. If the 'standard' warranty were considered unacceptable at the outset it would be appropriate to develop the warranty model to include a recommended spares holding based upon MTBF, or more onerously full replacement of the product range used including mobilisation costs. Either way both parties are fully aware of expectations and obligations from the outset.
Contractually speaking if the warranty document is deficient you are really only left with negotiation. An option being to approach the supplier for access to the original product production drawings and look to have bespoke replacements made.
As a compromise and considering the actual failure rate seems quite low (5# out of 1500#) - though in no way trying to belittle the frustration you must feel - are there visual aspects where use of the replacement product may be viable and use the original products from those locations for the more visible locations ?