My opinion differs quite a bit to Maxsteam on this issue but this could be because I am a mortgage broker myself.
To save another 5% deposit could take several years depending on the house and the area you are living in and once you have saved the 5% you may even need to save this again if the house prices have risen. You are on a road to nowhere if this happens. Your landlord / friend may want to sell now or may not offer you the same deal or discount again. if you are able to buy now i would certainly do so. To wait another year or two will almost certainly cost you with rising house prices...albeit only 1% - 2%. I would look to buy now!
I do agree to speak to a local mortgage adviser to see what he suggests but I would also get in contact with some local housing associations, they often have plans and shared equity schemes whereby they will buy the whole house and then you will then buy a portion of this back from them (up to you how much, normally max purchase of 90%) and you will then rent the remaining portion from the housing association.
I know that Maxsteam is not a fan of this kind of scheme but I am as I feel that for many many people this is the only way they are going to be able to get a foot on the property ladder. It can be done at reduced cost and reduced deposit levels.
As an example, we have a local lender (Northern Ireland) that will accept applications with no deposit (100% mortgage) if it is going through the local shared equity scheme, and they will even honour the shared equity element as a deposit so if you took a 70% share and your local housing association take 30% they will honour the 30% as the deposit / equity and you get a 70% loan to value mortgage product instead of the usual 90% or 95% (big difference in interest rates). This is down to the lender and not the schemeholder but hopefully you may have something similar in your area.
Speak to a local mortgage adviser and see what is available in your area