A currency tailwind at Barclays, owing to a weaker pound-dollar exchange rate, has prompted broker Investec to reiterate its positive stance on the banking stock.The broker maintained a 'buy' rating and 295p target price, saying that "Barclays remains our preferred UK bank"."Although the London-listed stock has recovered strongly, up 14% since the market's 16 October trough, outperforming all other UK banks since then, trading on just 0.8 times 2014 estimate tangible net asset value (tNAV) it remains the cheapest UK bank," said analyst Ian Gordon."Indeed, despite outstanding legacy issues, we think Barclays is now a much more resilient bank, with materially strengthened leverage and capital positions, while in simple valuation terms, the collapse in £/$ suggests to us that it hasn't really re-rated at all!"The benefit from a falling GBP/USD will be seen in the bank's tNAV per share metric which is forecast to rise from 287p in the third quarter to 292p in the fourth. It should increase further to 335p by 2017, Gordon said."The FX tailwind remains a key support for earnings too. Our 2015e forecast for Investment Banking revenues (in Sterling) is +3% year-on-year, but the anticipated gain is entirely attributable to our expectation of an 8% fall in average £/$, with, in the view of our Economics Team, potential post-election political gridlock in the UK posing further downside risks."Barclays' shares were up 0.2% at 234.5p by 10:27 on Monday.