What if I pay for expenses personally, can I still claim against the business?

Any expenses personally paid by you are still tax deductible through the company assuming they are business expenses.

You should treat them as expenses and can be paid back to you tax free from the company. Alternatively you can keep them in your director's loan account to extract at a later date.

You should obtain and keep any receipts incurred on behalf of the company as these will still be needed.